JRT,

Thanks for you prompt reply and advice.

This has been coming on with more persistency for a couple of weeks. Steady deterioration.

Just a quick reply;

No unusual noises.
Have run CC clean.
Ran scandisc.
Ran Grisoft AV prog.
Have to reboot from the wallplug.
Happens on any programme at any time but seems to prefer online activity like YouTube and email log-on.
OS is XP SE. About 4 years old.
No error messages.
According to DxDiag; 275MB used 973MB available.

Here's a possible clue; The monitor often clicks back to yellow standby light from green when booting up and have to start again until it holds on green. Might be monitor failing? When picture freezes there may be a band of coloured broken lines about 1-2" deep

Lucky to write this far!