I recently upgraded to a Jetway 830ch mobo c/w 900 Athlon and now my system is very unstable. For no apparent reason it will reboot while I am in the middle of doing something infact anything! Using the built in hardware doctor that came with the mobo the only thing that seems unusual is the -12v which is running at -13.5. the shop where I had it fitted asures me that this is normal. Sometimes when it reboots it will lock up during memory testing and I have to reset it manually. Any help would be appreciated. I have 256mb of ram and a 20gb hdd.

whats the temp of the cpu when it turns off?

chance is u have a crud heatsink and fan

A poor quality HSF could cause the symptoms you describe as Steve rightly suggests, but I don't like the odd 12V rail output either. Suggests to me your PSU may be iffy - is it an approved AMD model (see the AMD website for approved models). I may sound like a broken record for saying this as often as I do , but Athlons are rather hungry when it comes to power consumption, and a bad unit can cause all sorts of bother. 300W is the minimum size needed, and if you can afford a 350 of 400W unit so much the better.

GM

It sounds expensive but a good quality Enermax PSU will set you back around £35-40 but its probobly worth it for piece of mind alone.

I would also suggest that if the PC shop which did your upgrade didnt reformatt your hard drive when they stuck the new mobo and CPU in that it can then cause problems. I found this with my PC.

yep just fixed my mums pc with eactly the same problem. i tried everything. and as luck would have it, the second to last bit of hardware that i tested was the offending party ie the power supply. pc kept rebooting very regularly.
Get youself a good enermax power supply. you can get em all over the place. try
scan.co.uk
overclockers.co.uk

bothj in my experience very good companies to deal with.
I've received many orders off them both with never a problem to tell of.