The problem - occuring after 15 minutes - tends to indicate a possible heat problem...take a room fan and allow it to push additonal air through the backside (or front) - (there are also - cooling "zap" compressed cans - but not for you) It could be the CPU - or the motherboard - or a chip

IF IT IS HEAT - It can be perhaps resolved with a second fan inside the case - (it fixed my similar problem six years ago for about 15 bucks.)

The heat problem could be as simple as the "grease" on the chip's mount.

Sincerely,
Gargoyle

Are you comfortable taking the cover off your case?