Sorry it didn't work for you. For the moment I do not have anything else I can think of. Maybe take a break from it for a day or so, just to help dissipate some of the frustration.
It would help if we knew someone with a 4870 that is currently running MW.
Oh, a thought..... OK, you said Collatz crunched fine. Have you tried a couple of Moo wu's? Again, just to see.
Edit: Not that you need to do this now but when you feel like it run GPU-Z. This utility just gives detailed info on GPUs.