Lol!
Overheating seems like a reasonable explanation, gonna try prime95 now
Spec:
Amd Penom(tm) II X4 b55 (4CPUs) ~ 3.2GHz
4096MB RAM (DDR3)
AND Radeon HD5750
So you mean 70 khash/s, not 7 000 hash/s...
Try to manage 190 kh/s
https://litecoin.info/Mining_hardware_comparisonsome configs out there