I have not read the updated posts since this morning but as I was logged out I wanted to mention/ask this...
Is this a project started in X3 that you opened in Platinum to continue working on or is it a new project started in Platinum?
My quick answer for both scenarios are...
If it is the former, finish the project in X3 and if you could maybe start a test project in Platinum and report back whether that fresh, started in Plat project, behaves in the same way.
If it is the latter then yeah, something is screwy.
Long winded blathering...
When I upgraded to X3 I had been working on a project in X2. I tried to continue the project in X3 because I wanted to use some of the new goodies. The project would not play all the way through. It would get stuck at a certain point and I'd get a dropout. It refused to play beyond that point... amongst other issues. There was no rhyme or reason to it. It work fine in X2 (as well as anything really could in X2 which drove me nuts). Any new projects I work on that were started in X3 however were perfectly fine and more stable than anything I had ever previously experienced with Sonar (I bought in at X1 Prod Suite).
I finished the project in X2. Annoying but whatever.
That said... I have more or less successfully transferred my main X3 project into Plat without any noticeable problems so far. However It is mostly audio based (only MIDI drums) and all my plugs/synths are 64 bit (and actually came with X3... so aside from Addictive Drums 1, TH2 (full) and Melodyne it's all Cake stuff (and those are Cake included/"approved" tools). Also all I've really been doing is tracking and some light editing.
I did however start running into some problems with a CWB that was created in Plat, sent to me and opened in Plat. There were a lot of things that could be causing that though. Frozen synths (that I do not own), lots of MIDI tracks, missing plugs, the project went corrupt after loop recording (which is notoriously screwy and my HDDs aren't set up correctly making things worse) and... well it's a CWB which brings up other possible issues.
Anyway... as I said if it's an X3 project I, personally, would just wrap it up in X3 then move on to Plat for new stuff.
To anyone concerned James may be trollin... that is very doubtful. He's opinionated but AFAIK he's a true pro and contributes a lot (although usually in the Songs area). I am not feeling well right now and not smart enough to solve problems for a dude like him so I'll let you all do the troubleshootin'. Just wanted to toss those little nuggets into the fray.
Cheers.