There were some false starts and some incorrect information (my fault), but the problem (and the solution) were found in Guitar Rig. It is in this thread, here and here. Thanks to Dan, Bob, Spencer, Joseph and Craig for your help. I would have been utterly lost without it.

 
Sorry, I didn't know how to title this in any way that would make good sense. Here's what's happening: In a project I'm working on, with audio and MIDI tracks (but only about 20 tracks altogether), the lead electric guitar track will play one time only at the level I have set for it. On the second and all subsequent plays, the level drops drastically -- maybe 12db -- to nearly inaudible. The same thing happens if I stop the playback and restart it. There is volume automation on the track, and the fader in the console appears to be following the automated levels, but what I hear (on every playback except the first one) is very low level, not even close to the level I have set. This is just an electric guitar track, straight audio, played through Guitar Rig, with some reverb added.
 
What's weird about this (to me, anyway) is that on the first playback when I first load the project, all the levels, including this one guitar, are normal. But when I restart the track or try to play it a second time, the level on this one instrument drops. And, of course, all the other instruments and voices are working normally.
 
Anybody have any thoughts on this?
 
EDIT: If I close the project without saving, I can play it upon reopening it, and it works -- one time only.

Is MIDI involved in any way?
If you bypass Guitar Rig so there's nothing in the audio path, does this still happen?
If you turn off automation read, does this still happen?

Anderton
Is MIDI involved in any way?
If you bypass Guitar Rig so there's nothing in the audio path, does this still happen?
If you turn off automation read, does this still happen?

No MIDI on this track. Same thing happens with Guitar Rig bypassed. [EDIT: As Craig Anderton points out, this seems to be incorrect.]  Same thing happens when I turn off the "R" button on the console fader. Thanks for jumping on this, Craig. I'll be away from the studio for an hour, but I eagerly await any other ideas you may come up with.

Does it decrease more every time you play, or do one major drop and stay there?

One major drop and stays there. I have the volume envelope visible in the track, and it doesn't change visually from the first play to subsequent playbacks.

More information, although so far it means nothing to me:
 
There are two electric guitar tracks in this project, and I realize now that they are both behaving this way. There is volume automation on only one of the two. The other four "pure" audio tracks (3 vocals, 1 percussion loop) are working fine, as are all the MIDI tracks (Addictive Drums, Session Drummer 3, DimPro bass and Sound Center clavinet).
 
Both the guitars were recorded using basically the same setup, and I am now going through the settings, trying to see what may be causing this. After the precipitous drop in volume on these two tracks, the settings in the console view and the track view look the same as before to me. The tracks that are acting crazy are not in any groups or assigned to any bus other than master. They do have reverb sends set up, but so do some of the tracks that are working normally. I don't even know if I can delete those sends, or if it's worth it to try.
 
Still hoping for ideas from y'all, though.
 
UPDATE: I have been tweaking the mix all along as I record this project, so I tried exporting a mix. I had already played the project, so the guitar tracks were in their low-volume state, but the stereo mix came out fine, both guitars up where they belong. But they are still almost inaudible when I play the multitrack.

Create a new track.
Ctrl-drag the audio which has problems to the new track. 
Mute the old track.
Does the new track exhibit the same behavior?
 
Also, does the volume drop happen before or after Guitar Rig? If it happens before, if you're using an amp with distortion there would be less distortion. If it was afterward, the tone would remain the same - only the level would change. Even assuming GR isn't involved, this would be useful information.
 
Apparently it's not Guitar Rig, because when you bypass it nothing changes. Ditto reading automation. So let's isolate it to raw audio playing through a virgin track and see what happens.
 
One more thing: When you re-start playback, is it always from the absolute beginning of the song? If you feel like doing something random to see if it makes a difference, create a copy of the project, select all, and move all the clips right so it opens up a couple measures at the beginning. What happens then?
 
 
 
 
 

Anderton
Create a new track.
Ctrl-drag the audio which has problems to the new track. 
Mute the old track.
Does the new track exhibit the same behavior?

 
I dragged the rhythm guitar (the one with no automation) into a new, empty track, and the level seems to be correct, so yay! But the lead guitar track (the one with many pieced-together clips and lots of volume automation) did not want to be dragged. When I grabbed it and started to move it, the only thing that moved was the automation envelope. I could not move the clips with either the smart tool or the move tool. I'm sure this is in the manual somewhere. If not, I'll be back asking you about it tomorrow.
 

Anderton
Also, does the volume drop happen before or after Guitar Rig? If it happens before, if you're using an amp with distortion there would be less distortion. If it was afterward, the tone would remain the same - only the level would change. Even assuming GR isn't involved, this would be useful information.
 

 
There is some GR distortion on both these guitar tracks, and I can still hear the distortion even at the super low level.
 
 

Anderton
One more thing: When you re-start playback, is it always from the absolute beginning of the song? If you feel like doing something random to see if it makes a difference, create a copy of the project, select all, and move all the clips right so it opens up a couple measures at the beginning. What happens then?
  

 
I have restarted playback at various random places -- the very top, a few measures in, halfway through the song -- and the same thing happens each time with these two guitar tracks. Since dragging one of them to a "virgin" track seemed to fix my problem (and it's bedtime), I'm not inclined to experiment further tonight, but if there's anything to be gained I'll try messing around with a copied project tomorrow.
 
Craig, I want to thank you for helping me on this. Late on Sunday night! I'm grateful that this forum is here, and that you (and a few others, you know who you are) are so willing to share your knowledge. I hope some day I can be useful to someone here.

Asking questions is useful, because it elicits answers that will likely pertain to others as well.

Anderton
Asking questions is useful, because it elicits answers that will likely pertain to others as well.

In my many years of working with computers, I have learned that it's better to get the darned thing working than to know why it stopped working. Your answer got me back in business -- thanks again -- but in this case I can't help wondering what caused this goofy problem, and why this workaround, works.