i have a 91 rs 5.0 and i had a radiator tank crack so i replaced the radiator and my car will heat up realy quickly to almost over heating and then jump quickly back to about half then will slowly climb back near redline and seems to run about half when the heater is on but once i turn it off it goes to about 3/4 to over heating then drops quickly to half then goes back and repeats i have replaced the stat the fan motor

 

Since it happened after the radiator replacement you might want to take a look at that. It maybe plugged or something.