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

[backini]

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

[backini]

the cpu is usually around 70% when it starts. One thing I have noticed is that there is a box to tick to lock the clock at 44.1khz in the hardware page of the delta contol panel for the soundcard. I've noticed that the box gets unticked somehow. If there are issues between the powercore and the audiophile does anyone know who's at fault? I hope its Maudio as I still have my receipt, are there any issues with RME cards that anyones aware of? I've been told the converters are top notch.