The 300rpm fluctuation is the TC locking/un-locking. The trans computer is VERY sensitive to the FIPL voltage, as you've already noticed. I recommend replacing the FIPL, it's cheap at most auto parts stores. There is a idle speed screw on passenger side of IP, but you hafta rotate the throttle shaft to get at it. Also, the FIS (Fast Idle Solenoid) plunger is adjustable (1/4" open end wrench). It might be out too far at warm idle. Set the idle screw after turning in the FIS plunger. Then when engine is cold, reset the FIS plunger to get the rpm you want. Check the electrical connector sequence to IP. If FIS is connected before the CES, the FIS will always be on.
To check the connections to the IP, here's what you need to do. Turn on key to 'run' position. Check the main wire that comes from the loom to the IP (injection pump), does it have juice. The wire runs to the FSS (fuel shutoff switch) connector first, then to one side of the Cold Engine Switch(two-prong sensor, located in hole behind the upper radiator hose connection). When the engine is cold 'juice' will pass thru the sensor, if the sensor is good, and go to either the FIS (fast idle solenoid) or the CTA (cold timing advance), then to the other. If the engine is cold and sensor is good it will be 'closed' and allow 'juice' to get to the FIS plunger which holds the idle speed up. If FIS plunger is working, but not pushing up far enough, use a 1/4" wrench to turn/extend it until the idle speed is ~1000 rpm. The CTA (cold timing advance) near the spider/distribution end of the IP 'advances the timing' to make engine run smoother/run cleaner/less smoke. The Cold Engine Switch opens/shuts off the electricity to the FIS and CTA after engine reaches 112 degrees, and engine speed/rpm will/should drop to ~650 rpm. If these three items are working correctly, when you go to start engine, hold the throttle pedal at least 1/2 way to floor to allow the FIS plunger to 'set' itself. Use a test light to check for electricity/juice at each of the solenoids, FSS, FIS and CTA. Let me know if this helps. Good luck, OkieGringo