Believe it or not E-Bay or Alibaba can get you in touch with international suppliers that have reputable products. (since thats where they're made anyway nowadays) Just be sure to check their rep.
But he other two users are spot on, there are apps out there available at places like download.com that will run your PC at diffferent speeds to determine if overheating of the CPU is the problem.
Are you getting a Blue Screen of Death or just a power shutdown? If just a shutdown I definitly suspect the CPU, easiest fic for that in the short term is gettting some fresh thermal paste and reseating the fan and heat radiator. Or just upgrade, they are inexpensive.