The reason it doesn't come back down is because it is getting too much
air. The thermal timer doesn't have anything to do with this and the
throttle switch could only affect it if somehow it is keeping the main
butterfly from closing all the way.
I would check the linkage and also make sure the screws that hold the
butterfly in the throttle shaft are there and good and tight. If they
get loose the butterfly shifts position and sometimes doesn't close all
the way.
I don't remember what the throttle return spring is like on the D-jet
but that might also be worth checking. If tapping the pedal didn't help
to bring it back to normal idle, I would look for vacuum leaks but since
it does, it really points right to the throttle butterfly going all the
way closed.
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George Downs, Bartlesville, Oklahoma, Central US