Mark- Did you figure out the cause yet?
I wonder if you could maybe have a loose throttle cable jacket clamp or just have the cable adjusted so it doesn't easily return the TPS to 0% position/fully closed throttle before the control hits its stop.
Next time it happens, with engine "idling" at 1000 rpm, you might try disconnecting the cable at the engine throttle lever to see if RPM drops back to normal 640-650 ECM-controlled idle.
An ECM scan of the TPS, IAC and various sensors could also help reveal anything amiss there
