It's over the last week & the psu has been in for oh, however long it was when I came here & asked advice on swapping it over - a few months back? Since I posted this morning, the system has hung & recovered without rebooting twice & each time it's given the 'vpu has stoppped responding to driver commands' message - so I'm pretty sure it's the graphics card driver.
I'm going to roll back to the old driver & see how it runs. The box is used for email, accounts, general work stuff, so my guess is I've put in a driver that's nicely tuned up for Castle Wolfenstein or whatever & at the same time, not very suitable for Sage & Outlook.