At stop or signal a/c on,in gear idle speed drops to 500 with annoying shimmy on 1994 Volvo 940

Volvo tech performed throttle body clean & idle adjusted, injector service, BG upper valve clean, new air & fuel filter, IAC & AMM Cleaned. When you stop idle is about 700 but then you see it drop to a stable 500 but shimmy starts. Increase idle screw enough for 100 RPM’s at stop, in gear, a/c on, when engine is turned off and restarted RPM is 1300-1400 & snatches heck out of tansmission putting into reverse. Any suggestions?

Asked by for the 1994 Volvo 940
big no-no to try to adjust idle speed as this is handled through the ecm. everything needs to return to factory settings and see if problem persists(probably not). if it does persist check to see if idle drops when cooling fan comes on(probably so). if that is the case then fix will be an updated e-prom as the is a voltage drop to iac when fans operate.
original setting applied. Idle is ok in gear @ stop, foot brake applied & radiator fan engaged. Problem is only when a/c engages, of course the radiator fan engages simultaneusly but the additional drag of the a/c drops the rpm's to 500. Idle is stable idle @ 500 but for the shimmy develops due to the drag of the a/c at that low idle. Turn off a/c or take it out of gear, either option, and the idle is ok at 700-800 and no shimmy. I'll consider the e-prom as you mentioned.
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