I'd first want to rule out fuel fliters or a leak in the fuel system, as that's the most likely cause.

Is this happening with the A/C on? If so, if you roll the window down and try accelerating while next to a stationary object (like a concrete barrier on the interstate) can you hear a clicking sound? If so, check the A/C cut off switch on top of the valve cover (it's part of the KLIMA system that is on the w123s for '85 only). On my '85, the switch became intermittent with age and would cycle the compressor on and off under moderate to heavy acceleration, causing a surging feeling and making a clicking sound. It would get worse as the temperature under the hood increased (I guess causing the old plastic to swell).

Good luck.

__________________
Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013