l have 1999 mercury mystique with manual transmission. Car runs great except every time when l release gas pedal, rpm drop below 500 rpm and the engine shuts off. l had that problem before only duringrainy weather. It seems like problem started again with cold weather. Could somebody tell me what could be wrong?

Possibly I.A.C. Idle air control valve

l would first check air filter and clean throttle plate. After that you need someone with an OBD2 scanner that can moniter the IAC, TPS throttle possition sensor, O2 sensors, and such to verify which one or combination is causing your problem.