Okay, I have a 1970 cutlass s with a 350 rocket and AC. Im getting ready to lay behind the car on a hill and let it finish me off, lol! The overheat idiot light kept coming on, so I did the following:
-put in a three core aluminum radiator
-put in a new thermostat(twice, 180 degree vice the 195).
-new water pump
New lower hose(nice and stiff)
-installed a temp gauge (twice)
What I got from both gauges is a crazy fluctuation from 190 or so to 260 in less than one second, and then back down to the 200 or so range. When I stop and shoot the thermostat housing with a temperature gun it says it is at about 200-210. What the hell is causing this?

 

You have an air pocket in the cooling system. You need to work it out, I usually stuff a water hose into the radiator filler neck with a slow flow of water with the engine running.

 

Yep, air in the system. I once had a slow radiator leak and I knew when the coolant level got too low as the gauge would spike as you described.

 

 

Be sure the temp sending unit is actually submersed in water at all times like on top of the manifold. If it is not you will get strange readings.

 

Thanks fellas, Im gonna check that out today!!