Automatic Transmission

[Reischmann]

We have a 1970 280SL, automatic transmission, with 54,000 original miles.
Drive it regularly, mostly short distances on the weekends.
Went to the store this morning and it ran and shifted perfectly (as always)
Backed out of the parking place, shifted into forward, and the transmission started to slip. All forward gears do the same. Reverse works perfect.
Transmission fluid is clean and full.
Any ideas on what could have suddenly changed?
Thanks,
Mike

[ja17]

Check the fluid level on level ground with the engine running. If it is ok, check to make sure that the transmission three position solenoid or its linkage is not sticking. Remove the access panel on the right side of the tunnel inside the car to see if the linkage if stuck.