Okay...for the benefit of other forumites, Guitarmech111 sent me a PM with the project.
 
Fixed!
 
First of all, did you know the guitar and bass clips are not the same length (the bass clip is correct, the guitar is longer)?
 
Second, the bass clip was not trimmed and it seemed to have some other metadata or weirdness, perhaps from trying to loop before it was trimmed properly. You always should make sure the loop is the right length before trying to loop it. So here's the fix:
 
1. Select the bass clip. Choose "Apply Trimming."
2. To make sure it's a "clean" clip so you can start over from a suitably initialized state, select the bass clip and choose "Bounce to Clip." This will return it to a standard WAV file.
3. Select the clip and choose Ctrl-L to loop. It will lengthen to 16 measures, but no worries.
4. Double-click to open the Loop Construction Window.
5. Set Beats to 48. Tempo will indicate 85 BPM, and you're good to go.
 
The reason I knew it hadn't been trimmed properly was that the first time I tried to loop it and I set beats to 48, the BPM was 84.175 or something like that. That's why it was going out of sync.
 
I didn't have Superior Drummer, but set up SD3 and used your MIDI track. I've been looping the bass and drums for 15 minutes and everything is in sync; FYI that's with the metronome off, so I'm not convinced that you have to have the metronome on for things to be in sync. I think it has a lot more to do with cutting to zero-crossings instead of measure boundaries.

This is not really solved. The drums are staying on time, but the audio is creeping further out. I have the new groove clips stretched out to measure 99 in my project. I have measures 39 through 50 set to loop. playing back that range over a dozen times, the sync gets off still. If I stop and restart playback I can tell the difference. At least the groove clip issue is resolved. :) Now it is the loop playback out of sync issue.
 

And you turned off Snap to Audio Zero Crossings prior to setting loop points, right?
 
I'm sure this is the kind of thing I could solve if I was sitting at your computer, but I'm not. I don't know what to recommend because I've never had this problem, so I've never had to solve it...so I can't recommend a solution.
 
I am sort of curious, though, why it's so important to be able to loop a section internal to the song. Why not just copy and paste the loops to fill the needed amount of time? And if they go out of sync then, you'll know the loops aren't trimmed to precise measure boundaries.

Frankly, I think the whole clip/loop boundaries thing is a red herring. I've been playing around with this for the last hour or so. I ginned up 4 lanes of MIDI driving Addictive Drums for 8 bars with a two-bar repeating pattern. I recorded all parts live, and quantized 50% just to tighten things up a little so the clips still started and ended randomly.
 
I then bounced each lane's output to a separate audio track, and started playing around. Addictive Drums doesn't null like Session Drummer due to sample randomization so I didn't try to do that; I just let the parts play together and relied on hearing any sync errors that were more than just phase variations.
 
First I tried every randomly sized loop imagineable with snap disabled, changing lengths, dragging them back an forth endlessly and clicking randomly inside the loop boundaries to change the Now time, then stopping and letting loops play for dozens of iterations without moving them. The only problem I managed to generate was having the MIDI notes drop out a few times (maybe once in 20-30 moves of the loop), but this would clear up as soon as I moved the loop range again. Nothing ever went out of sync.
 
Then I enabled Audiosnap with Autostretch on the audio, and started alternately changing tempos and randomly sizing and dragging loops. No problem.
 
Then I disabled Audiosnap, groove-clipped all the MIDI and Audio and dragged it out to measure 217 or something like that. I repeated the random loop sizing and dragging exercise, and could not generate any real problems. One time when resizing a loop, I managed to get MIDI and audio to go out of sync, but it was a fixed offset - not a drift - and cleared right up with the next loop move.
 
I don't understand why some environments seem to have this problem consistently, because I find it extremely difficult to replicate any thing resembling a MIDI-Audio sync drift, no matter how I abuse loop start and end points. The only thing that really matters is to make sure that groove clips repeat at whole measure/beat intervals, even if not on the beat, and that's easily accomplished by slip-editing their ends to the nearest measure/beat before groove-clipping them.
 
I dunno. It's a mystery.   Makes me never want to upgrade my machine or OS again.

brundlefly
 
I dunno. It's a mystery.   Makes me never want to upgrade my machine or OS again.

If you do decide to upgrade your machine or OS, you can use whatever I'm using. I don't have problems either.

Anderton
And you turned off Snap to Audio Zero Crossings prior to setting loop points, right?
 
I'm sure this is the kind of thing I could solve if I was sitting at your computer, but I'm not. I don't know what to recommend because I've never had this problem, so I've never had to solve it...so I can't recommend a solution.
 
I am sort of curious, though, why it's so important to be able to loop a section internal to the song. Why not just copy and paste the loops to fill the needed amount of time? And if they go out of sync then, you'll know the loops aren't trimmed to precise measure boundaries.

I am absolutely positive that I have zero crossing off. After what we went through, I don't forget easily.  ;)

As I said, with that setting, it is better sync, but eventually goes south. With the metronome on, it is rock solid.
As far as importance of looping a set of measures, if the program is able to do something, it should do it with excellence.

I have the progression looping because I try to get inspired with timing and phrasing of word passages. Things don't come to me instantly. I have to put some effort into making sure stuff fits in phrases and thought patterns to make sure the words make sense and are somewhat of a storyline. That is just how I roll.

The loops are trimmed to the precise timeline, beats and tempo. The GCL values are 48 even beats and tempo is 85 even. As I mentioned before, I will not soon forget what you helped me with. I don't like shooting myself in the foot if I don't have to. My machine is plenty fast so latency is not an issue.
 
The good part about this is that I can restart playback to get back in sync or run with a silenced metronome. I guess it is wrong to expect a program to work without issue if it has an available function?  ;)

I really don't think this is a snap/audio crossings issue. I have seen the problem just looping a single MIDI drum track with no audio clips involved, and making sure the loop is of the exact right length in the loop module. It still drifts off without the metronome on. I saw this back in X1 too.

I think Sonar still has some MIDI timing issues that need to be ironed out. Another thing I see occasionally is when a note that falls exactly on the beat doesn't sound when you start playback from that beat. I confirm its position in the event inspector and it's right on the beat. The only way I can fix it is if I move the note away from its position and set it back down again, as if to remind it where it should be, lol. Nothing I can reproduce to file a bug report though.

I have the same problem. It was the same in X2.

Hi Folks,
 
Those of you experiencing this problem, are you using instrument tracks or split MIDI/audio tracks for your synth playback?

C'mon guys, the cavalry has arrived and is awaiting instructions!