My setup is similar to yours and I haven't had those problems. I have an X2 4800+, 2GB Corsair TwinX DDR, and an XFX 7800GTX. But I'm running on an Epox 9NPA+Ultra.
Maybe it doesn't like your high overclock?
Do you like avast? I've been running AntiVir 7 for about a week, and I like it less and less. I love the interface, but it has some annoying tendencies. Maybe the Premium edition would clear things up, but I don't want to pay $ only to find it has the same problems as the free version.
MadMax