Engine stalling

Starts great, runs fine on high idle cold engine. As engine begins to reach operating temperature, idle speed will reduce marginally. Shortly thereafter engine will dramatically slow down and stall. Occasionally, engine will "recover" itself and return to normal idle but will quickly repeat slowdown and stall. At operating temperatures engine will stall as soon as it is shifted into any gear. Can keep it running (stationary) at operating temperature by keeping foot on gas pedal so it rums at higher speed. Can sometimes force it to "drive" (a very short distance, length of driveway) using two foot method but engine hesitates and sometimes backfires and then stalls.