Normal in town, cold on highway.

[Ingvarg]

My ´95 XJR (AJ16 supercharged) runs at normal heat when in town. Gauge in the middle, fans on when supposed to, heater works and everithing normal, but when at higher speeds on highway the heat drops significantly. Between 1/4 and almost down to zero on gauge, and then back to normal when back in town.
I guess thermostat stuck open ??
Second opinion and/or other ideas would be greatly appreciated.

[Popsjag]

I had this problem on my n/a AJ16 3.2, earlier this year.

I changed the thermostat and also the 2 sensors on the thermostat housing (only because the coolant was drained below the height of the thermostat, whilst replacing it).

I've had no issues since.