have you gone in to bios and reset all the bios setting to default (f5 on asus or load optimal)?
do you have problems getting it to fire up?, pressing power button multiple times?, if yes,i would suspect the psu as mine did this and died slow over two weeks.
if not, try turning down everything in bios you can, the cpu-nb down a level and do the same for the ram.
see if that brings back stability.
could you post a shot of hwmonitor or something so we can have a...