check to see if you have your fan attached to your cpu or have cooling paste applied to it, that can initiate a failsafe

If the problem only occurs while attempting to run Sins and not any other games, i doubt the problem is overheating. Even if the cpu's contact to the heatsink is flawed, this game is not CPU intensive what so ever, its more memory intensive than anything else. What are the hardware specs of your system? include cpu, cpu heatsink, ram and quantity, motherboard, video card, hard drive(s), psu and what operating system (ie. xp, vista, 32 or 64 bit). Is your cpu overclocked? video card overclocked? Im running sins on a custom built rig and the only time i ran into problems running sins was when i had my 4870 1gig overclocked to the highest stable settings i could reach. I turned em down a notch and now everything is good to go. If your system isnt overclocked, first, make sure all your hardware is mounted correctly and securely, specifically ram. Also, completely uninstal sins and any files associated, defrag, reinstall and update to the latest version. I had to do as such once or twice.

Its not an overheating issue.Iuse artic silver thermal compound on all my builds and have monitored the cpu temp closely it never went above 34c.The game crashes when I go from the load screen to single player.i have ran memtest several times.there is no problem with the memory.My system specs are as follows.

Motherboard - Foxxconn NF4SK8AA-8EKRS(Socket 939)

CPU - AMD Athalon FX-60(dual core)(Dual Core Optimizer Installed)

Memeory - 4gig Dual Channel DDR2 400 OCZ

GPU - Nvidia EVGA GTX 260(216 Core) 896mb(Latest Drivers)

Sound Card - Creative Audigy ZS(Latest Drivers)

PSU - 850 watt Corsair(75 amp single 12volt rail)

DVD - LG DVD Burner/Light Scribe

Hard Drive 1 - 160gig Western Digital ATA 133

Hard Drive 2 - 75gig Velociraptor(10,000rpm)

Hard Drive 3 - 75gig Velociraptor(10,000rpm)

Hard Drive 4 - 500gig External(Storage)

3.5 Floppy - Generic

Typically a spontaneous reboot is either due to overheating, power issues, or a BSOD. BSOD being the most frequent culprit, do you have windows set to show them or just automatically reboot? If the latter, turn autoreboot off and you should get the BSOD and be able to get some idea of the issue (generally hardware or driver related).