Probably the temp sender but here is another inexpensive thing to try. Clean out the tranny cooler in the radiator by disconnecting both lines, using the pressurized tranny cooler spray can that connects to the cooler. Available at your local auto parts store. I also recommend having the radiator rodded. Lot cheaper than repaired and cleaning does not help on ones our age.

87 Raizer, 4.3V6, Mod TH700R4,231CHD, detroit rear and LSD front, bouncy seats, 2" lift and 32x15 BFG MT tires.