First to language, if youi have a 1984 280CE as you state, and not a 1974 280CE then you do not have EFI (electronic fuel injection). You do have K-jetronic. If it were a US car it would have had K-jetronic w/lambda and would have had a small amount of electronic control. The european 280CE had plain K-jet with no electronics (the idle air might have been electronically controlled but not fuel.
I would look for a vacuum leak or a defective decel dump valve.
Bosch Master, ASE Master, L1
33 years MB technician