Hey Fuddbeer,
If you haven't resolved yet...To help this forum help you, I would suggest that you provide some details of your system. To do that, please review this thread and provide details of what hardware you're using as well as a SANDRA report:
http://duc.digidesign.com/showthread.php?t=193636
There are several possibilities on why you're getting these errors, including the fact that maybe your PC is just not fundamentally compatible with PT's. Thus, you may have to so some overall upgrades to better your PC's performance - hopefully they'll be minimal, but may want to prepare for the worst (just being honest). Regardless, please provide your hardware details and SANDRA report.
Also, I've been reading that the problem could be that maybe you just need a dedicated audio drive. I know you stated you tried everything, was this one of them?
To answer your last question, I actually had to do a co-install of PTLE8.0.4 & PTMP8.0.4, since I use an M-AUDIO interface (DELTA 1010) & a DIGIDESIGN interface (11 Rack). One thing I know for sure is that you must have the same exact versions of PT's in order for them to work seamlessly. Keep in mind, when you install you have to start with PTLE8, then PTMP8 and then install each update in sequence (PTLE8.0.1, then PTMP8.0.1 and so forth). It's time consuming, but well worth it in my opinion.