It sounds like the solenoid simply is not able to over come the return springs if it only moves the linkage enough to take up the slack in the linkage. If the solenoid is operating correctly, it will move its armature to the limit of its travel. Try pulling out the armature by hand to see if it advances the linkage further than when electrically activated. Also, try pulling out the armature when the solenoid is electrically activated. My guess is you will find that
the solenoid has enough travel to raise the idle speed but if not then there is a linkage wear or adjustment problem. If the solenoid armature can be pulled out enough to advance the linkage, put a voltmeter between its screw terminal and ground. Measure the voltage available when in reverse and when in one of the forward gear positions. You could also try jumping +12V directly to the solenoid to see if the armature moves out enough to keep the idle speed where it belongs. If a good source of +12V (via a jumper) will not advance the solenoid adequately than the solenoid itself is weak. If +12V does advance the solenoid but the available voltage at the solenoid is appreciably under +12V when being supplied by the pressure switches on the transmission, than one or both of the pressure switches is bad. There is a separate pressure switch for reverse and for the forward gears so if the system works well in one but not the other transmission position, the corresponding pressure switch is bad. Of course, it is wise to also check for damaged wring. If memory serves, the idle is specified to not drop more than 50 RPM when the transmission is moved from P or N to one of the drive positions and this is how I used to adjust my car. Unfortunately, these parts have become very expensive. The transmission pressure switches are 001 542 13 17, current US list $162, my price $119.36. The constant speed solenoid is a 000 072 05 00, current US list $398.00, my price $292.86.
