In addition to what Ka2H wrote, you may be having power supply problems
(either house current or power supply in the computer), overheating or loose
connection problems. Check all external connections to the computer and any
and all devices to make sure they are firmly seated. If no loose
connections, open the case (hopefully a desktop type not a laptop) and check
for quantities of dust in the system. If so, blow it out with a vacuum's
exhaust or compressed air (DON'T SUCK IT OUT WITH A STANDARD VACUUM.) Make
sure that all connections in the system are secure. Pull and reseat all
add-in cards and memory sticks (if you feel comfortable doing so and after
you have grounded yourself.) Connect all the cables again and turn it on
with the cover removed. Do all the fans turn (don't physically touch
anything in the system with the power cable plugged into the power outlet as
all newer computers provide power to the motherboard with the switch turned
off.)