You've hit on a problem that I had been dealing with for some time. I have Zeniths (I'm assuming that Stromberg and Zenith are of the same Stromberg-Zenith line) on my '71 250C and had the same problem which took several tries to fix. Many things were wrong with mine so I can't say which was the one and only culprit, but here is the "top ten" list:
First of all, with an older car, I had a problem with tank sediment getting into the bowl and clogging the idle jet. To solve that, I cleaned the bowls and installed a second, aftermarket, replaceable element filter (the clear kind where you can inspect the filter). This type of filter appears to filter a much smaller particle.
Secondly, vacuum leaks were a major problem. I rebuilt the carbs myself and found that the throttle and float housing butterfly mating surfaces were slightly warped. Ask your mechanic if they were checked or, if you do it yourself, check the true with a straight-edge. Another part that may have been causing a leak was a warped metal heat shield between the insulating plates between the carb and the intake manifold. I replaced mine because hammering them out just will never work. I would bet that the vast majority of my problems were caused by vacuum leaks in this area.
Thirdly, I found that after fixing the above, the fast idle screw on the front carb (I don't know your model, so I'm assuming 2 carbs like mine.) was adjusted too fast. That is, the primary throttle plate was open too far at warm idle. When I reduced it, the surging diminished and was able to proceed with further tuning (an art in itself). My fast idle was probably advanced to counter the vacuum leaks...
Lastly, the carb float level may be off. On a hunch, I suspected that as the fuel in the bowl shifted, it caused the surging, so I went to the next thinnest washer under the inlet valve. I don't know if this was really a problem since the previous set-up was to specs, but the problem hasn't returned and no signs of rich mixture or leaking as of yet.
I'm assuming that your tech did a complete rebuild and all passages are free and clear.
Beyond those "baseline" repairs, the next step was tuning. Again, depending on whether your model has 2 carbs or 1, it can be easy or difficult without meters. I have tuned mine thanks to input from the Tech Forum, Haynes, etc., but can't say if my methods have been Kosher so, that said, I should probably leave this to someone more experienced with your model to go further. In any case, I really recommend getting your hands dirty and get to know this carburetor - she will either be your best friend or your worst enemy and you've got to know how she works in order to decide which it's going to be. I have learned more about carburetion thanks to my MB than I ever thought I would (and am still learning daily!).
A last note: I'm not a MB mechanic and don't pretend to be, and may have missed things that are apparent to professionals - I only know my car and its problems, but I hope I have helped. Their responses to this post will help me as well.