Erratic temperature gauge (once again ...)

[aphelion]

Took advantage of the dry weather at the weekend. Temperatures not much above zero, but the temperature gauge behaved like a yoyo - once warmed up (about horizontal position of needle, 'cos so cold outside) it started rising until almost into the red, then coming down again - so quickly you could watch it moving! Then up again, and down, and up etc etc etc Movement of needle was clearly visible (?too rapid to relate to fans cutting in above 90 degrees - which they certainly were when we were stationary - can't hear them over the sports exhaust when driving!), and needle continued fluctuating over about a quarter of the gauge - about 20 degrees variation in temperature or more. Sometimes it seemed to alter when going uphill, or downhill, or even cornering (like fuel gauge used to before it stopped working) - but not exclusively so. Other times gauge reading went right up when coming to a halt at a junction, other times not. Didn't come down when pushing on after dawdling through speed limits, as it used to. Never got into the red. Water/coolant and oil levels all OK (and not mixed as far as I could see). Could this be one of those intermittent gauge problems that are such a b**gger to fix, or something more worrying? Anyone else had the same problem? This is certainly not how the gauge used to behave, it was always predictable - if a bit nail-biting when sitting in jams during the summer! Funnily enough the fuel gauge worked OK for the first 15 minutes yesterday - first time in about 1500 miles!! Grateful for any ideas/comparisons of V8S gauge behaviour. Thanks.

[aphelion]

Thanks guys.

once the engines warmed up the temp gauge reads fairly steady at horizontal during normal running, rises when stationary (i.e. idling at lights etc. before the fan cuts in and it then drops back slightly) and it also rises when the car's getting a good caning!

This is the normal behaviour ours used to exhibit - although ran a little hotter after our independent dealer retuned engine to deal with pinking. Use of 15W50 was some help there over standard Mobil 1. However, radiator pressure cap used was 13lb. We replaced this with 15lb cap just about time temp gauge started mucking about - so went back to 13lb thinking this might have had influence (logically it shouldn't surely?).

When heater was on last w/e all dial faces and the dash panel got pretty warm (in addition to usual steaming up) - is this typical, or indicative that temp. gauge isn't lying and overheating possible? No other signs, and car appeared to be running well.

Connections might well be the answer - we drove over some pretty awful bumpy roads on the IoM recently (in addition to the velvet smooth TT course ) and suspected something would shake out ... Will let you know how we get on - in a while, car isn't garaged at home, so not immediately accessible. Thanks again for help.