If the car starts to "cut out" at highway speeds when accelerating, there is a near 100% chance that the pressure switch which screws on top of the intake is at fault. The way to check if it's working is to unplug the electrical connector from the overboost protection valve on the firewall and take the car for a drive. If the symptoms are not present, replace the pressure switch on the manifold. Or, if the symptoms do not cease, plug the valve back in and unplug the switch to check if the valve is at fault. Good luck.

__________________
Regards,
Aaron

My '84 300D started to shake at idle. I repaced the motor mounts which did not help. I turned the rack damper pin in a few times and now the engine runs smoooth. Always nice to know another trade secret. Repair shops have it made with this repair, making the big bucks by the turn of a pin.