asrock motherboard

I have a asrock 775VM8 bd with celeron D 2.8 chip. 1gig ddr ram
My problem is many lockup and shut downs.
I've had this problem with 2 different hd and new 512 ram cards,\.
removed cd and floppy drives and allcards so only running
hd ram chip mb
voltage is good were checked with 3 meters hook up
this happens any time on boot up or after 6 hours.
chip operating temp 46c
could this be motherboard problem and if so what is a good
replacment for home use?
Thanks