In case anyone is still having this problem, I though I would mention this: I was recently at a MB dealer looking at a 300SE they had for sale. In the records there was an invoice with the statement "why does the motor shake?" The repair section of the invoice stated "replace fuel pump check valve." I don't know much about the gas motors, being a diesel guy, but I thought this might be of help to those that are suffering from this problem.
'84 300D, 172k