Hi ohnolook, thanks for your response, and good to know I'm not alone.
I'd heard of issues with Flash Player and browsers (specifically Chrome) in Win 7 64-bit, but have eliminated them by completely uninstalling Flash, and since experiencing a freeze with no browsers in use.
I've also eliminated the Windows Update Service as a cause by making that manual only.
CPU temp seems fine, all fans in operation, no signs of overheating.
Seems unlikely there's a RAM fail if Memtest and Prime95 run with no hiccups. Can't see what motherboard issues there could be.
Happened before I installed any custom drivers
, so unlikely to be a driver
In summary - still stumped, but I'll let you know if I discover anything else.