Im gona try to explain this, I had 5870 also and sounds like same problem. Like does your computer when you first turn it on any day cycle multiple times before it actually goes into windows? And when you do make it into windows sometimes the screen changes colors or seems like resolution tries changing? Sometimes it freezes/blue screens and reboots? Takes about 15min till its "warmed up" and you dont have problems?
Idk let me know go into more detail if you can if im not describing it right.
This is exactly what is happening with my card