Sudden CPU issues with Soft Synths

I have created multiple projects with a lot of soft synths in them over the past few years (mostly Z3TA 2 and Dim Pro 64) : a few weeks back I upgraded to X3C when it released with no issues whatsoever. About a week ago these projects started distorting horribly on playback. I noticed that the leftmost core in the CPU meter is spiking into the red within seconds of playback starting : after a while the project tempo slows and playback deteriorates even further. Freezing the synths helps to a degree but there is still some distortion : in fact I've found a couple of tracks with only one or two synths that exhibit this issue. I've added no new hardware (using an M-Audio Fast Track Pro USB) and the only software I have installed recently is a new driver for an M-Audio Axiom 25 MIDI keyboard, which is not even connected when all this happens. Any suggestions would be appreciated.

Thanks for your response : I'm not currently running anything but X3 so I can't check. I had ruled out X3 as the cause since I had no problems with the initial release or patches b and c. In fact I've never encountered such an apparently out-of-left-field issue with any Cakewalk product. I could try installing an older rev, how far back would you suggest I go? I'm running W7 64-bit.
 
A.

Thank you. The forum heading on my PC just says Sonar Producer and Studio, didn't realize it was version-specific.
 
I'll look into all of your suggestions but I suspect the Fast Track Pro may be the culprit : I don't recall the driver asking me about the Windows version and in fact the M-Audio site doesn't have the driver anymore, I had to email their support for the latest. If one subscribes to the philosophy that there are no coincidences, the fact that this happened around the time I installed another M-Audio product- the Axiom keyboard- may be important. There's no driver rollback on it so I reinstalled the last Fast Track Pro drivers and it 'repaired' them but who knows. What interface are you using now?
 
Further to your suggestions regarding the multi core and core parking toggles, why would my problems not have surfaced before if this were the case? I've been using the same machine with Sonar for almost four years now....8 G of RAM, four core, all four cores are graphically represented on the CPU meter but only the first spikes into red. The machine's always worked quite well up until this.
 
Thanks again.
Alex