XLN Addictive Drums + SONAR 8 : Hearing distortion?

I've got a fairly busy project I'm working on - CPU use hovers around 50-58% usually - and I've been trying to record a Bass track (actually Chapman Stick but whatever) and I'm hearing distortion on the previously recorded drum tracks.

It's not in the samples. When I was tracking the drums, they sounded great. When I was playing back an early mix, they sounded great. But when I've armed another track and are actively recording another instrument, I hear distortion. It's most noticeable with cymbal crashes.

I tried "freezing" the drum track to see if this would cure the problem, but I'm still hearing it.

Has anyone else who uses AD or similar have this problem?

Possibly related observation: I have noticed that during playback, the first note from a softsynth can be distorted by a white noise transient. It's like the softsynth is sleeping, then suddenly receives a MIDI event and wakes up with a transient burst of static. I don't hear this on mix-downs, just "live" playback or record. I mostly notice it with TruePianos.

Again - have other people noticed this? 

I've only noticed it on busy projects (but then I've really only worked with the one project for months at a time so i don't know if it is fair to generalize).

Has anyone experienced this, but had the problem go away when they upgraded their DAW to a fast processor, more RAM, whatever? I consider 58% to be "working hard" but I don't see why that would introduce noticeable audio artifacts. 

Anyway, thanks in advance for any thoughts or advice.

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Pentium D Dual Core, 2.8 GHz, 2 GB RAM, Windows XP
Sonar 8.5.2