Hi Chris,

From your discription it sounds like the idle stabilization system is
reacting to a varying input or the control module is just faulty. If you
haven't already done so remove the R/R tail light and pull the idle
stabilization module from it's mounting and pull it out far enough to
check each pin value against the attached sheet of test values with a
voltmeter. Check the top of the module or the multiplug for pin # I.D.
If any input value varys it can cause the module to cycle the stabilizer
valve and upset the idle speed causing your symptom. Also make certain
there are NO intake air leaks and that the O2 sensor is functional and
fuel pressure is OK.

Regards,    John