Arster... [ Jack ],
Thanks Jack... I am happy your problem was so easy that I somewhat accidently helped you find it as quickly as you did !!

By your following my request to write your POST above give as much detail as you could, I finally realized that your dirty orifice was the "in-line" orifice and/or " Y " connector supplying vacuum to the transmission vacuum modulating valve on the tranny... so the dirt was starving the "Tranny-Shifter Vacuum Control System" of vacuum thus allowing the Vacuum Control Valve [VCV] that sits on top of the IP and bleeds off vacuum according to throttle position... and this allowed the VCV to drop the vacuum level too quickly which in turn was causing early up-shifts you experienced!

A final recommendation... install cheap [ less than $2 ea ] in-line paper-medium [low friction loss ] filters to keep the dirt out of your system... on your model, install one of these on each vent line [ labeled "a" on the diagram ] and usually terminating under the dash.
One of these comes from the VCV [#65 in diag.] and for those of you with an EGR on your engine [post-1981 models!?], the other comes from the electric switchover valve [#81 in diag.] that controls the EGR incorporated as part of your Tranny-shifter Vacuum Control System on these later models. My 1980 and Jack's 1981 240D do NOT have EGR(s) system despite what these diagrams indicate!
Regards,