When I first bought our 86 XR w/ 97K miles on it 6 years ago, I had the same issues with the temp readings. I had replaced all the temp senders and switches and thermostat with units I tested myself in a pan of heated water and a candy thermometer, and it still was not reading right nor was the electric fan triggering. I tested the old switch and sender, and they gave nearly identical readings and switch point that the new ones did....hmmm.
As part of the initial maintenance go-through on this car, I pulled the intake manifold and found that the water passage in the manifold is a very narrow cavity and was grossly restricted with accumulated pasty crud. With a severely limited contact with the coolant, no wonder the sensors weren't sensing. I completely cleaned it, reinstalled it with new gasketing, and did the following to the system (be sure to port the manifold when you have it off, ed):
First I did a normal system flush and backflush with a commercial Prestone product, garden-hose fitting, and modified Helms manual procedure as listed in the archives. Then:
1. Pulled all the water hoses, tubes, housings, heater control valve, radiator, and shoved brushes/coathangers into every exposed water passage on the engine.
2. Back-flushed the radiator with garden hose pressure. (I make "adapters" out of duct tape)
3. Clamped the old top and bottom hoses on the radiator so they would stand upright with the radiator laying flat, poured 1/2 bottle of CLR (as seen on TV) rust/scale remover into it. Sloshed it around for a few minutes, then filled the radiator with boiling water and let it sit for 1/2 hour. Poured the mixture out into a catch pan, and did the garden hose flush again. (It thought the radiator was pretty clean before I started, but was amazed at the crud soup that came out with the mixture)
4. Back-flushed the heater core with the hose, clamped fill and exit hoses with scraps of the old hoses, blew out the core with compressed air, and put in a cup of CLR and filled the rest with boiling water. Let it steep of 1/2 an hour or so, drained the soup into the catch-pan, then thoroughly flushed with garden hose water. (have the exit hose pointing upward)
5. Soaked the metal tubes in CLR and hot water, brushed out with long handled, narrow test tube brush.
6. Reinstalled the radiator, new hoses everywhere, and back-flushed the engine block and fittings without the heater core or radiator in the circuit. (the Helms manual has a fairly good description of water flow in the system)
7. Installed all the hoses, and filled the system with distilled white vinegar. Don't laugh. (gallons are available at you local supermarket. 5% acidity) Ran the car till temperature was reached (gee, the fan comes on and the gauge reads up) and let it sit overnight with the vinegar in there.
8. Next day, run up to temperature, and flush the livin daylights out of it. (you can let this organic mixture onto the driveway, environmental impact is nil. The smell is interesting though)
9. Fill with 50/50 antifreeze and DISTILLED WATER. CHANGE YOUR COOLANT EVERY YEAR.
Note #1. I have been using Texaco brand antifreeze lately. It has an unusual bright orange color and has an "all metal" rating. Looks great in the reservoir.
Note #2. If the wife wonders what the heck your doing running back and forth from the stove to the driveway with great pots of boiling water, tell her your in the middle of a complex weather experiment. Buy your own vinegar.