Once old Veccy gets going, the gauge normally gets to 90deg - & stays there, but on a run today, I noticed around 15 miles down the road (it had been on 90deg) - it had dropped to about 70 - it stayed there until I could pull into a layby - I stopped the engine & had a look under the bonnet - everything looked & smelt! normal - coolant level OK etc (For a Vectra!) Restarted engine & it went straight to 90deg - & has been there ever since.
Any answers?