I have searched the forums for my problem,but has come up empty. During the day the temperature gauge on my 1997 Disco, reads fine, but sometime after turning on the headlamp at nights, the gauge gradually goes past maximum. Sometimes it stays there,other times it creeps back to normal. If I stop and turn off the headlights, with the engine running the gauge drops to normal. I 've fitted a new sensor,still the problem exists. The engine shows no signs of overheating,and there is no loss of coolant. Any ideas?

Not absolutely sure but it could be an earthing problem on the headlight circuits, try isolating the source by removing power fuses one at atime on RH, LH head and side lights, this may point to which lights you need to correct earths to, if it's not that then there may be a problem with the instrument lighting behind the dash.
The temperature gauge is fed volts when you turn the ignition switch on and the reading of the gauge is controlled by a resistance device (the sender in the head, RTD), if there is a condition in the inst panel where a backfeed is occurring or an earth is off, turning on the lights could send the gauge on its upward track.

Had a similar problem, temp gauge would go up to max and go down when handbrake applied. handbrake cable would heat up and smoke. Fitting a new engine to gearbox earth strap fixed it. Check all the earth connections, clean them up with wire brush etc. 2 in engine bay, near battery. Engine gearbox strap (underneath car).

White_Disco said:

Had a similar problem, temp gauge would go up to max and go down when handbrake applied. handbrake cable would heat up and smoke. Fitting a new engine to gearbox earth strap fixed it. Check all the earth connections, clean them up with wire brush etc. 2 in engine bay, near battery. Engine gearbox strap (underneath car).

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+1
Had a similar problem , check your earths. to verify , try using a jumper cable to replicate an earth lead.

As above, an earth problem - I.E. the problem being there isn't one, somewhere, and the temp sender supply is just finding the easiest way to earth..

Jump lead idea is a good one, should solve the problem instantly - so you can then do a more permanent repair.

Thank you all. Took off the instrument panel today, and found that the earth lead to the panel was not making good contact.