Crackling Then Distorted Audio, is it powercore or audiophile? Options

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When I play back a busy arrangement in Logic, after a while I start to get slight distortion, then crackling, then severe distortion over the audio. I suspect its some sort of conflict between my powercore element and my audiophile 2496 as this started to happen on my G4 just before I upgraded, now I have a G5 and I've started to get the same problem although the only thing in my machine that isn't new is the powercore pci card. I would put it down soley to the powercore were it not for the fact that when I switch the audio drivers to inbuilt audio, the problem disappears, pointing at the sound card as being the culprit. I have followed both TC and Maudio's advise (re-installing latest drivers, moving around to different pci slots) but had no luck. Any help would be much appreciated as I've spent days scanning forums for similar issues.

previous system: G4 dual 867, 1 gig ram, audiophile 2496, powercore element, Logic 7.1
Current system: G5 dual 2 gig (pci-x), 3.5 gig ram, new audiophile 2496, powercore element, UAD-1, logic 7