It seems to disengage, ease off the throttle?

It sounds like you're experiencing a problem with the transmission on your 1987 Mercedes-Benz 190D. The symptoms you've described, where the transmission disengages briefly and then re-engages when you ease off the throttle, could indicate a few potential issues. Here are a few things to consider:

Transmission Slippage: Transmission slippage occurs when the transmission momentarily loses traction, causing a brief disengagement of power. This can happen due to worn clutches or bands within the transmission, low fluid levels, or hydraulic issues. Even though you've replaced the ATF and the filter, if the clutches are worn or there's another internal problem, it might not fully resolve the issue.

Vacuum System: Many older Mercedes-Benz models, including the 190D, use vacuum-operated systems for various functions, including the transmission. A vacuum leak in the system could cause intermittent disengagement and engagement of the transmission. Check for vacuum leaks and make sure all vacuum lines and components are in good condition.

Throttle Linkage or Cable: If there's a problem with the throttle linkage or cable, it could affect the transmission's engagement. Make sure the throttle linkage is properly adjusted and functioning as it should.

Transmission Control Unit (TCU): The TCU is responsible for controlling the shifting of the transmission based on various inputs, including throttle position and vehicle speed. If the TCU is malfunctioning, it could lead to improper shifts or disengagements. However, please note that the 1987 Mercedes-Benz 190D might have a mechanical transmission control system rather than an electronic TCU.

Torque Converter: A problem with the torque converter, such as a faulty clutch or a lock-up solenoid issue, could cause the transmission to disengage and re-engage unexpectedly.