1997 Mercury Marquis overheats then not?

[AChernak]

Hi,

Please tell me what the most likely problem would be for the following symptoms.

The idiot temperature gauge (while on the the highway) goes up and pins to HOT.

Then, if you you let up and drive slower it seems to come back to it's normal middle setting. It continues going up and down though (even if you drive slower all the way). It just does NOT pin to the HOT setting when driving slower (so far).

When it pins to hot there is NO HEAT. The heat comes back when the gauge goes back to normal.

From personal experience, what would be the most likely cause of this problem.

Thermostat.

Heater Core.

Water pump.

Radiator.

Clogged hose?

Thank you for your help. I'm looking for a technician's experienced opinion so that I can decide if I should fix or limp along and look for another vehicle.

ACC