Testing these systems is not as easy as it should be. Do you have the factory shop manual i.e. CD?
There are step by step procedures for testing the systems and it is not very darn easy to test them without starting at square one and then moving through the test procedures one at a time.

If I remember though behind the acc control unit there are several pods that control various functions at various settings. Reading the schematic for these reminded me that I should have paid more attention in shop class but if you can isolate these to their various functions you may be able to exchange one with another and see if this corrects what seems to be erratic behaviour.
Sounds like you may have a pod that is not working correctly or maybe more than one.
Do all the vacuum controllers cycle as you are working through the various settings? Foot vent actuators, dash vents, etc?

The other potential is the dreaded acc servo on thsi year and it seems to be a mystery device as I cannot find any true tests for this unti other than work, no work...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
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'85 300SD Silver - Sold
'78 Ski Nautique