Make sure the secondary ignition is solid.
(wires, plugs etc...) Then remove and clean the ISC valve, little tube looking guy on top of the intake, clean with some decent carb cleaner. Put you thumb over one hole, fill it with cleaner, plug the other hole with you other thumb and shake. Do this untill the cleaner comes out fairly clear. Also clean the sensor plate, just blast it and be prepared for a hard start.
If it persists, check the lambda duty cycle, it should be about 50% fully warmed up, pin number 3 in the round connector on the l/h fenderwell, set your meter to duty and ground the other lead. 3mm allen in the a/f sensor, clockwise to richen.
Also check for vacum leaks if lean, this will cause the idle control to fail to "catch" the engine as its returning to idle.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.