Thanks to Srdjan and Adam, here's an update...
CPU and case fan OK, GPU is fanless. Memtest86 reported only 4GB not 8GB but no errors. Tried booting with one stick out, OK. Moved that stick to other slot, still OK. Tried other stick by itself, no boot. So I guess that's faulty. Odd though because I tried that a few days ago with slightly different results. One stick wouldn't boot like today but the other threw UEFI into panic mode with frantic beeps from the mainboard.
Then replaced good stick by itself and rebooted. Ran OK for a minute or so, then hung when I clicked reply in T'bird to send this! Sometimes it hangs the plasma screen but I can [ctrl][alt][f2] to get a CL login and kill plasma from there but sometimes (like now) even that wouldn't work. Beginning to think one stick is completely f****d and the other one on it's way out.
I think I'll try some new RAM and if it's still unstable then maybe a mainboard and CPU too...
Thanks for the help
--
Phil