You should try updating the BIOS, before you drop the big bucks. I currently also have Q6600, and it works like a charm.
Don't use ASUS's updating software. Use DOS to update the BIOS. Its safer, more reliable, and faster. Try updating your BIOS to 0703, and see if it works.
As for 50% usage, I'm suspecting its a code error in OS. I'm not sure though, but if you can, and have the will and energy for it, try to reinstall the OS.