Ok, this is a hard to describe accurately... I have upgraded to X2 recently, my computer's well up to spec and has done the job without any problems in X1. Good audio interface, everything running smoothly, plenty of CPU power. I'm using drum loops with great results. It's safe to say I'm using a fair few tracks in my projects but have never had any issues up to now. I've been spending a fair amount of time mixing to get a template finalised to move on to other songs. Suddenly the drums decrease in volume for no apparent reason. It's like they've been cut down by around 20db but the cymbals move up and down in volume too. At first I thought it might have been because the CPU usage was reaching a limit, but I'm not even close to that. I haven't overloaded on the FX either and I can find no trigger that causes the problem except that it happens when I'm recording guitar and am nowhere near the drums at all. But it only seems to happen on the drums and nothing else. The drums don't have any FX on them apart from some EQ.I checked everything in case I accidentally hit a wrong buttong but no. And if I solo the drum track it plays at the correct volume, so there's definitely something weird going on. I thought I'd solved it yesterday by reimporting the sound files into a new project and starting with a clean template. It worked for a while but it's just happened again and I'm baffled. It's definitely not a computer problem or anything to do with the audio interface. Any ideas?

What kind of drums and tracks are involved? Recorded drums? Hardware synth drums, softsynth drums? MIDI drum loops, Audio drum loops? Your own recorded MIDI? Step-sequenced MIDI? PRV-drawn MIDI?

We'll need to know the end-to-end signal routing to have a reasonable idea where the problem might lie.

When does the drop in level occur, and for how long? Same place every time? Is there any automation on any tracks?

They're audio drum loops, 16bit ACID WAVs and I've never had any problems in the 2 years I've been using them. There's no automation on the drums either. I place the files into the tracks and that's it. There's a BOOST in the group bus bin which is already a part of the template. That's the only plug in on the group. Some of the individual drums tracks have some EQ and that's all. Once I've placed the drums in I always bounce them to clips to cut down on CPU usage. And I do the same thing with the other tracks once I'm happy with the recorded audio. When the sound goes down like this it doesn't come back and stays at that low level across the entire project from there on. Re-saving or restarting the machine doesn't solve the problem which is there on the whole project from start to finish so once it happens the only thing I can do is re-import everything into a fresh project and start mixing all over again. And it's Sonar Producer X2 I'm using on Windows 7. As I said, Sonar X1 worked this way without a problem so it may be something that's X2 specific.

Suddenly the drums decrease in volume for no apparent reason. It's like they've been cut down by around 20db but the cymbals move up and down in volume too. 

You probably have some sort of automation enabled. Unless the volume changes are printed in the audio file. Its one of those 2 things. Volume changes have nadda to do with your CPU getting kinda high or your PC specs. Its a gain stage issue.

Assuming the level of the bounced waveform is not visibly losing amplitude, I suggest you start by putting a pre-fader send on the track direct to Main Outs, mute and solo the track, and see what you get. If it sounds normal that way, the problem is somewhere in the chain between the audio clip and Main Outs. If it still loses level, the only thing I can think that could cause that other than a bug might be Clip FX.