Temp guage climbs to high nml then a minute later it drops back to low nml

A lot of these vehicles will stop the car when the system senses a major fault, get it checked out.

Sounds like the thermostat is sticking, on warm up on lot of vehicle the gauge goes past normal and when the thermostat opens drops back. If it is going higher than normal, the thermostat is opening late. Needs changing before it stops opening altogether.

Souds like it's way low on coolant. The heater will blow cold air when the cooling system is so low that there is no water (coolant) running through the heater core. Then the thermostat opens, it trys to be normal but the system is low so it gets too hot in just a few minutes. If its that low, there must be a leak somwhere.

First question is what constitutes "overheating".
Since the car has been parked for years, the thermostat could be sticking closed.
The cooling system could have air pockets and need to be bled.
The water pump may be corroded inside and not pumping enough volume.
If the gauge goes above half way, the fans should start up. But half way should be more or less normal, not hot yet.
I would look at flow problems first, beginning with removing the thermostat and driving it without one for a few days. Also check and see if there is a process to bleed the cooling system of air.

Any chance of a head gasket leak? Is the coolant level still full? These were known for head gasket issues. Coolant is a closed system and level will not drop unless there is a leak. Make sure there is no air in the system. Replace the radiator cap to make sure that it is operating properly.

Probably a stuck open thermostat or the thermostat is the wrong temp range for your vehicle.

get yourself a stilson wrench and remove adjuster bolt then sort out the belt tension.......(you could also weld a nut to the bolt head to give you some leverage.)

change the thermostat and the temp gauge in the radiator

Check to make sure your thermostat works right by monitoring the warming up of your engine. With a completely cold engine, start and run the engine for a few minutes, then put your hand on the top radiator hose to monitor if and when it gets warm as the thermostat opens more and more and note the gauge reading( if it works) the top radiator hose should be unbearably hot within 5-8 minutes. If the therm. opens right it should close right aswell, staying open a little while eventhough the radiator may have cooled may be what you are experiencing causing the rapid cooling and is nothing to worry about. The Hose should not get warm very soon after starting the engine. Regarding you gauge, lets take one step at a time, verify proper op. or the therm.

My mechanic in our town did a quick computer check. Car computer indicated a low level of antifreeze, he topped it up, cleared all messages off the car computer and said to watch the the heat guage for any further irregularities. I'll keep an eye on it, love the car, great mileage fun to drive.