involuntary change of parameter pro-53/logic

I had two tracks on Logic Express 8, one running Absynth, and the other Pro-53... suddenly I noticed the pad I had created on Pro-53 sounding completely distorted... it took me a while to figure out what it was, but I realize that now and then, the Envelope Amount from the Filter Section on Pro-53 kept changing, randomly...

Anyone encountered that? What gives? I have one of the new MacBooks running on Leopard, 2Ghz dual, 2 Gigs Ram, blablabla...

Not a big problem (I'm just beginning with all of this anyway) but still, just trying to figure out if there's anything I can do about it...