The ICM regulates your idle settings, so if it's me, I would put on hold the solenoid idea for the moment. I would not want to introduce another variable when my problem havent yet been solved.

The disparate readings on your tech's hand-held computer could possibly arise due to a faulty ICM. Go have him test it again once you get yourself a new ICM. 500-600RPM in "D" mode is definitely within acceptable standards.

Mervyn

1983 W126 (280SEL)