RPM jumps up and down at 1000 to 1500

I had an experience with a 1984 T-Bird with a 302. The 1986 may have the same system. With that car there was a temperature sensor that affected the idle speed once the car was close to warming up.
With even later models there is a "cold start sequence" which is an electronic way of replacing the ancient choke. The "cold" feature set the fuel mix and the RPM and made starts easier. Now you did not say if the starting problem was when hot or cold or both.

The poor performance when slowing down can be related to the idle rpm being set too low once the thermal switch cuts out. What happens is someone tunes the engine for a rpm that may be correct for the car, but the thermal switch has not dropped the rpm yet. Then when the car really heats up, the idle drops even lower and when you take off your foot from the throttle, the throttle closes too much and the engine cuts out when you are slowing.

Put in a new thermostat. The idle settings can not be set until the thermal switches activate to a run position. Also you may have some vacuum lines crossed which are making the cold and hot cycles reverse. There should be a label with vacuum line routing shown stuck on a part that holds the radiator.

cold oil is thicker, higher rpm=more pressure .factory guage is actually working like it should. if worried hook external guage on oil galley plug and check oil press.manually.

This is normally the way it is suppose to work. You need to have the RPM measured for High RPM and duration. What you are describing is how an electronic choke is suppose to work.

The errors would be how high the RPM goes and for how long it lasts. When cold the RPM, fuel mix and timing are changed to give a rich fuel mix. As the O2 sensors warm(Could be defective) they signal the engine to lean up. The water thermostat also has something to do changing the engine to a lean running slow condition.

If obtaining heat from your heater seems to take Forever, the thermostat could be weak. This would coincide with the engine continuing at a high RPM longer because it is not warming up.

Plugs ,wires,coils.

Check the water coolant thermostat. If it is stuck in the "open" position you are heating up all the water in the coolant system and this will take time. The thermostat keeps the water in the engine until warmed up to almost normal running temperature, only then does it allow the coolant to enter the radiator.

check out the idle control valve first

the engine tickover is too slow when the engine is cold its running on cold start, ie:choke,, after it warms up the engine slows down this is what is causing it to stall in gear,, get some one to hold the brakes hard on and with the engine running in gear ajust the tick over till its reving at 750 - 850 rpm the revs will go up a bit when its out of gear,, before you pick a gear to go into push the foot brake down to stop the car lurching,,,

Buy a bottle of Techron Fuel injector cleaner. It's sold in many grocery stores and all auto parts stores. Pour the whole bottle into the gas tank when you can fill up. Drive the car and don't put more gas in until the car is almost empty. That should fix it

try replacing your EGR Valve, or your Oxygen Sensor

i'am no expert on this car, but i would chk. the T.P.S. (throutle positioning sensor) there also may be carbon build up on throutle butterfly, not letting it close completely when you take your foot off the pedel.