Dropping a cylinder at idle

[Tom400e]

Hi all,

Car is 92 400e with 136k miles. When car is started it runs liek a champ. After reaching normal op temp if I sit at idle it starts running very rough and power goes way off. As soon as I get the rev's up over 3000 it clears and runs strong and clean again. If I get caught at anothe rlight it drops the cylinder again runs rough as a cob until I get the revs up over 3000 and it clears again.

I suspect a sensor malfunction of some kind. I'm going to try and attack it this weekend ... any suggestions on where to start looking or testing for the culprit?

TIA