Computer turns off when starting a game up.....

It is either one of two things power or thermal. To have the system power off when loading game is a bad sign. So take one of your cards out and test each one in single with the other removed to rule out the cards being bad. If still unstable then it is likely power however it is also possible that the board is defect.

A 1KW PSU that costs little over £100 seems a bit of a scam to me and I wouldn't trust it as far as I could throw it, that and it only has 22a on it's 12v rail so I reckon if you tried pulling a 1000w from it a small fire would ensue.

I had the same problem when I was powering my system with a 700 watt power supply. Changed to PC Power and cooling 1200 watt and problems all went away.

By the way, I know the OP and he's tried running each card without sli, no difference.

In the occasion that it does work, the cards overheat to over 100 degrees Celsius very quickly, whether in SLI or not.

Your power supply is junk. Get a real power supply. I'd be shocked if that PSU could handle a single 460 in a system with a system with a fast processor and a few drives. Powercool PSU's are extremely overrated.

i was having the same problem that yours, i buyed a Corsair 850HX and everything is going fine right now. I think the 850HX can handle your system, even in SLI. This PSU is one of the best you can buy!

I'm not familiar with that brand..... over seas and all. Says 66amps...... try moving the wires around. You may be trying to draw too much power off 1 "rail".... I'd try uninstalling and reinstalling drivers too. Power supply may not be the greatest either ???

i have done a dual power supply setup, where i have made a dual 24 pin adapter cable and connected two power supplies together, my main power supply is the powercool modular 1050w and the slave psu is an ace 620w, i have the 1050w powering up my motherboard and 8pin 12v rail and 1 GTX 460, the 620w powers up the 2nd GTX 460 and an SSD,HDD,DVD and 2 x 12cm fans... now my pc doesnt turn off... but my graphics cards still get red hot with SLI or not   i am planning on getting liquid cooling but i want to find out why there are getting so hot while air cooled ? specially in under 15 minutes