Did you ever fix this problem, or find the source of it? My car has this exact same symtom. It seems to occur more the day after it rains (maybe a result of the dampness causing a short somewhere). I am having my throttle actuator replaced as we speak, due to a surging idle/low idle/stall/etc. problem, and hopefully the problem that you describe will go away. I had my wiring harness replaced a year and a half ago, so hopefully the throttle actuator will fix it.

Matt
'94 E320 158k