I'm having this same problem with a remote machine. The MW ps_sep... tasks fail out after just a second or so.
BOINC 6.12.34, catalyst 12.1, win 7 x64 with a 4850.

I found a 4870 on craigslist and I'm picking that up in a few hours, so I will find out if it's the card.

It seems to be the 4850 as Al stated, but I can confirm that this afternoon.