I have a very similar situation with my MSI 4800SE.
After a couple of months working oc at 320 / 635, this card started to "dislike" a cold start. Usually it hangs the pc at post or hang during gaming
if I donīt let it warm a little, say 10 mins or so.
I have tested it in several pcs, I own 4 at the moment and I'm building my 5th...
Always the same thing...