Temperature Gauge Erradic

[Sigi]

On my diesel Land-cruiser 85, FJ-B60, on occasion very rarely temp. gauge jumps up for a split of second, then goes back to normal. It happens usually at high speed at low RPM.
Does anyone knows what could be the cause. My mechanic doesn't have the answer, he put in new thermostat, but the problem persists...

[David*All-Trac]

When you drive it, the temps climb and climb until the thermostat open... the the cold collant from the rad make the temp drop... so the gauge come back to the middle again.

Buy a mech gauge at any car part shop and put it in the "elbow" where the the thermostat is (remove the big square nut). Old gauge can sometime freak out !