It sounds like you might be going in and out of decel cut-off. I don't recall what all is in the circuit on a 420 but I would verify at least the operation of the throttle idle contact.

To verify the problem you could disconnect the EHA once the car is warm (won't start well cold with it disconnected) and drive it. If the problem is gone then the decel function is a likely candidate.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician