In reply to a post by cheshire_man:

I'm continuing to try things, but still getting BSODs and the occasional freeze.

Some recent observations:

The PC froze today, as usual the mouse laser went out and the whole system was dead. Screen still ok but no visible disc activity or response to caps lock etc. As it happened I had headphones on at the time and as soon as it froze I realised that there was a hum through the headphones, similar to a mains hum.

Any diagnostic significance?

Having earlier today (after the freeze) reseated the memory, the graphics card, and one of the power connectors (couldn't get one of them off so didn't do that one). Then maybe an hour or so later the screen flickered and a message box appeared at bottom right saying the graphics card/driver/subsystem (can't remember which) had stopped and had recovered.

Any diagnostic significance?

I'll be talking to the local shop that built the system in the next few days, I have a very good relationship with them. I'll be asking if they can try a different graphics cards (any VERY safe recommendations?); similarly try a separate LAN card and disable the onboard LAN chip (again, any recommendations?); possibly even try another PSU.

Any other constructive thoughts gratefully received.

I still think its psu related. Just seems simular to the problems i had which all dissapeared once the psu was replaced.

Worthy of an update now the problem is resolved.

Monday last week I took the PC to my local shop for them to have a look at.

They ran tests on memory - passed; CPU - passed; motherboard - passed; PSU - passed. At no stage did they get a BSOD (typical!)

Then they put their collective heads together and came to the conclusion that it was a memory problem. So to include the relevant bit of their email to me:

"We believe it was your ram modules that were causing your blue screen issues; your old ram was the hyper Tec value ram 1.65v in which we have now replaced with 4 GB corsair Vengeance 1.5v Ram.

The Sandybridge motherboard you have has a maximum voltage memory controller which can handle up to 1.59v.

We could probably tweak the motherboard to give more stability with your old ram but the best way is to simply replace the Ram to the 1.5v memory controller."

I've now had it back for over a week and it has run without a hiccough - previously the longest time BSOD free was 6 days (once) more normally 1-2 days.

So, in a week or so when I've got time, I'll rebuild to get a clean system - it's got a bit messy with diagnostic software, etc.

Thanks to all who offered their advice and guidance. Clearly the new memory would have cost somewhat more than the old so they lost a little but they know I'll bring more business, where they are reasonably competitive, because their service is so good.