Sounds like you have a great car! Has anyone removed the ignition distributor & replaced the "Trigger points" for the injection system or at least inspected them? The 1975 model was the last year for EFI & the system wasn't all that bad. The only bad thing was the catalytic converters on the exhaust manifolds!!! Keep the car original, have the system checked out by some old guy who was working on M.B.'S when your car was new!
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH