I have done a fresh install of Win 7. I have tested the ram sticks individually and together with Memtest86, they pass fine. All the bios settings are on Fail-safe so everything is running at stock settings.
I added the graphics card the other day because sometimes when it crashed it came up with multicolored lines across the screen. So thinking there could be something wrong with the on board graphics I added this card, and whilst I haven't seen the multicolored lines it still crashes a lot.
Sometimes straight after it's crashed or when I try and turn it on it emits long beeps, apparently this is pointing towards a graphics card problem but it did it when it was running on on board graphics and it does it now there is a separate graphics card in there too (about 30% of the time).
When Prime95 is running (it can run for 15 mins before crashing) with the processor is at full load the temperature goes up to about 70-75C, which is much too high I think. At idle the temp is around 35C. After a crash the event viewer usually shows this as the last error:
SELECT * FROM __InstanceModificationEvent WITHIN 60 WHERE TargetInstance ISA "Win32_Processor" AND TargetInstance.LoadPercentage > 99
I replaced the stock cooler with an aftermarket one and the idle temp is down to about 40C, with the load hovering at about 60C.
Despite this it still crashes often. I can get it to crash almost every time by running Prime95 for about 25 minutes without any faults, then stopping it. Within a minute of the test stopping it just locks up. I still sometimes get the continuous beeping on startup which I don't think is to do with a graphics problem.
I am testing each stick of ram in every slot in Memtest so I can totally rule them out, but I am really not sure whether the processor or the motherboard is at fault here.
When I first built it in April I fiddled around with the RAM timings a bit, but for the last 5 months all the BIOS settings have been on default. Because of the crashing at the moment all the BIOS settings are in fail safe mode.