temp gauge wont make its mind up

my old mans got a p38 dse doesnt get used a great deal (3.5k a year) and recently once the vehicle has got to normal operating temp (half way) a little while after it will go to the top of the white bit then go down and back up but if you turn the heated screen on it goes down instantly turn it off it goes back up is it likely to be a dodgy sensor / dash playing up / alternator etc:doh:. any ideas guys thanks

download rave, have eka code handy and check it works, disconnect battery and use rave to find and clean Earths. check plugs on back of dash
Also BCEM fuse box feed corrodes-clean it
While at it the bcem plugs sometimes corrode and are worth cleaning as a general precaution.

Use only electrical contact cleaner

Mine displayed this, dash power loss and weird things and the I thought I'd be buying a bcem repair-after the above=all fixed