Pc hangs up oftenly or show blank screen (by 2 users)

i have an d845gvsr motherboard and my motherboard sometimes detects the ram and sometimes it doesn't. when removed ram from the slot motherboard does not even give beep sounds. but after shaking the cabinet it detects the ram and starts working but after short time it hangs up or just show a blank screen and sometime at the boot screen just hang. then after pressing restart button, motherboard again does not detects ram. i have configurd everything from bios to default settings, changed ram, changed power supply, checked hard drive for errors. i am using windows xp pro sp2.