Engine temp drops when the a/c or heat is

You may ahve a bad engine coolant temp sensor. If it goes, the cooling fans default to always on high speed.

Sounds like the typical symptom for a failed thermostat which is stuck open.
After the car has operated for a while, check the rubber radiator hoses; if they are not hot and hotter, the thermostat needs replacement.

1998 Taurus. Heater does not blow hot. The temp dial is ok. When I switch to heater it continues to blow the AC

the thermostat can be easily changed from up top, the T-stat is located at the engine end of the upper radiator hose under the flange the hose connects to. Make sure u follow manufacturer's cooling system bleeding procedures when you refill after the T-stat is changed out.

have the fault codes read
the speedo gets its signal from the VSS ( vehicle speed sensor) and that sensor also reports to the transmission control module
So check for a problem in that area

Sounds like a stuck thermostat no heat water not circulating to heater core engine overheats stuck thermostat won't let water circulate through engine

if the engine temp is good (approx 200 degrees) you may have a clogged heater core. if engine temp is too low (less than 160 degrees) you may need thermostat

are the fans on when engine running

Just a piece of advice: could be the oil which lubricates the gears. Try changing your engine oil, maybe it has gotten a little too old and lost its lubricating properties.

You need to pressure test the cooling system. If it holds pressure, I would look at replacing the water temp. sensor. If it doesn't hold pressure, you have a coolant leak going on. If you don't see it dripping on the ground or inside the car, I would be suspecting head gasket problem.