Frequent lockups :(

[brian]

Is there anybody else out there using Debian Jessie 64-bit and XFCE on
an AMD PC who, for the 3 or 4 days, has been seeing frequent lockups
of their PC? The crashes seem to occur when I'm using a browser, and
using Iceweasel is worse than using Chromium. When it happens, that's
it, everything locks solid, mouse cursor won't move, the keyboard
doesn't appear to work, the display clock stops, the only thing I can
do is to use the reboot switch. The only thing I notice is that the
drive access light is still flickering (once every 5-10 seconds) and
there's been a disk repair needed every time I've had to reboot. 

I know there was an amd64-microcode package in the last bunch of
updates, it's for AMD processors other than mine (a Phenom II 6-core)
so I uninstalled that, with no apparent effect on the problem. 

The 'about XFCE' dialog gives the version as "4.10, distributed by
Debian". 

Anybody have any ideas, please? The lockups are happening every couple
of hours or so. 

Thanks.