I am still getting the random rebooting issue. The screen now flickers and cursor position resets itself before the system cuts off and reboots itself.Specs are:Microsoft Windows XP Professional x64 Edition1TB, 150GB, and 300GB Hard DrivesIntel Core i7 Extreme 965 @ 3.2 GHz12GB OCZ DDR3 1333 RAMATI (MSI) Radeon HD 4870x2 OC EditionGigabyte EX58-UD5 MotherboardThermaltake Toughpower 1200 Watt Power SupplyRAM tested and working properly. Motherboard replaced from defective ASUS board. Processor RMA'd and replaced. Main hard drive replaced and tested to be working, other two hard drives disconnected. Graphics card tested and working in ATITool. PSU tested and working.Any advice?

Have you got error reporting enabled, and have you set your system to not automatically reboot when there is a system error, so you can read the blue screen error messsage if there is one?Funnily enough I run XPx64 and get random rebooting issues too. I'm almost conviced its my RAM - my PC can run happily for days at a time, then out of the blue (pun intended), a blue screen error. I'm getting two new sticks of RAM today so hopefully I'll see if the RAM is the issue.

Have you got error reporting enabled, and have you set your system to not automatically reboot when there is a system error, so you can read the blue screen error messsage if there is one?Funnily enough I run XPx64 and get random rebooting issues too. I'm almost conviced its my RAM - my PC can run happily for days at a time, then out of the blue (pun intended), a blue screen error. I'm getting two new sticks of RAM today so hopefully I'll see if the RAM is the issue.

Yes and yes. However it still happens. I don't get a BSOD, it just cuts off and reboots.Once I had a RAM issue causing the same thing. However this time it is something else, now the screen flickers madly before the reboot.Even though ATITool showed no errors, I think the graphics card is malfunctioning. I am replacing it.

I have the same exact problem(no BSOD just freeze requiring hard reboot), I have the same specs as you but with 3 Nvidia 280 cards, at first I thought it was the software(FSX) so I installed FS9 and it happened again. Now I am replacing the ram with PC 1666 ram, hoping that will fix it. My machine is O/Ced to 4.0 Ghz but turned off HT so the temps are good with liquid cooling.I ran Memtest and got some errors so I am hoping this takes care of the problem.

My problem right now isn't a hard freeze, that was a problem that I had in the past though. PingPong, definitely replace the RAM.My current issue is that the Windows session just cuts off and the system reboots itself.

I say bad power supply. I've seen that too many times.Do you have a replacement to put in, to see if that is the cause?

What do I use to test the PSU?I don't have a replacement.

What do I use to test the PSU?

The only way I know of, is to use a voltage meter and load tester.

I don't have a replacement.

If it were me, I would1) get in case and verify that *all* cables, memsticks, etc. were seated 100% properly and were clean2) if #1 did not solve, I would get a cheap ($<80) replacement at a brick-and-mortar store, and return it if it doesn't solve the problem. You do not need anything near 1200 watts for what you're running.I'm still sayin': look at the power supply first

Ok I will try the PSU from my other system.

Sounds like a driver crashing to me. Check your event viewer's system message log and sort by column "type". Scroll down to where you see errors being logged and let us know what you find. You can find this utility by right-clicking on my computer and selecting manage, then event viewer, then system.

Sounds like a driver crashing to me. Check your event viewer's system message log and sort by column "type". Scroll down to where you see errors being logged and let us know what you find. You can find this utility by right-clicking on my computer and selecting manage, then event viewer, then system.

Yes that's a possibility too, thanks for mentioning that@PlainplanePost back and let us know what you find out with Event Viewer and/or the power sup.

I tried the PSU from the other system, and there was no change.I sent the card to MSI for an RMA, but since I now have reports of issues from other 4870x2 cards by different manufacturers having this issue, I too think definitely that it is a driver problem that ATI needs to fix, especially since ATITool reveals that the card itself is fine.To think I abandoned NVidia over an antialiasing problem only to spend $570 on a product that was unusable from day 1. Just my luck...