The next thing to check is the control pressure cable. Be sure that the thing isn't stuck out; the cable is pulled out by the throttle linkage behind the fuel distributor, but only pulled back by the internal spring force on the internal control valve.
If the cable is functioning the pan must be pulled and the control pressure valve disassembled for sticking.
The next step would be to pull the valve body and apply the pertinent clutch pack with air pressure. If the mechanical is right try a new VB
gasket and a proper retorque. Case/VB
warping and leakage causes considerable problems.
If none of that works then MB makes rather cheap rebuilt VBs.