I don't know if it could lock up your whole PC, but what about your vid card overheating? I had one once with a heatsink and cooling fan on it, and the cooling fan died one day which could have led to overheating. Whether it's the vid card or the CPU, I would guess it's an overheating problem since it happens only when playing games for a length of time.
'98 Camaro SS: Incon twin turbo. Sold
'99 Camaro SS: Heads/cam/bolt-ons. Sold.
'91 Camaro RS: Stock.
2010 Camaro SS: Hell yes.