You will need to do some forms of testing to get results. Does the car drive properly at 40mph? Does it accelerate well from that speed?
I would bump the timing to 15 deg BTDC (with the vacuum lines disconnected), at least till we have fixed the problem. The factory set-up is based upon having vacuum on the retard side of the distributor - many causes for this setting the timing too low: no vacuum, low vacuum, A/C on (no vacuum), vacuum retard unit leaking, corroded, malfunctioning, etc.
Once in this condition, determine if there is a single cylinder misfire or are all cylinders equally poor running. Effective cylinder balance testing can be done by cracking the injection lines loose one at a time (wipe spilled gas - be carefull) while noticing the difference in speed and vibration. Cylinders that are poor will give little if any change. This same test can be done by pulling plug wires. I personally would rather deal with the fuel than the spark.
Let us know what you find.
Bosch Master, ASE Master, L1
33 years MB technician