Intermittent distortion on audio

[huwrees]

I run PT LE 8.0.5 via an MBox2 mini, I had a new PC built in June last year, i5, 3.3ghz. I only use PT for editing and mixing as i have a large analogue rig for recording. For three months everything was fine, using iZotope plugins etc lovely. then in September, when i was playing back audio, it would sometime be distorted, modulated overall distortion. If i changed any of the Playback engine settings, it would go away, sometimes for an hour, sometimes for just five minutes. Again changing any setting in Playback engine would stop it. My cpu has never had more than 15% use on the largest session of editing/mixing. I opened a support ticket with AVID for £30 about two months ago, they still dont know what it is. I have installed PT 9 and PT 10 (they gave me a temporary licence) same issue. They thought it was the iZotope plugins (Nectar/Ozone). But it also distorts when i use TRacks3. The latest "solution" they have given me it to take the PC back to factory default and then reinstall all my software again. As i have made no changes to Win7 since i have had it, and i have already uninstalled/reinstalled PT LE 8.0.4/8.0.5, PT 9 and PT10, and tried different Mboxs but it didnt work. They don't seem to have a clue what it is but have been helpful in trying to find out about it! But i sent my PC back to the supplier they reset it and tested it. Came back and worked fine until due to an over current problem with a USB port i had to reset the BIOS. Distortion then came back, i updated the BIOS and there was no distortion for 2 hours, but now its back. the motherboard is an ASUS P8P67 Rev 3.0. Its now not overclocked just using optimised defaults. Anyone help with this?