I had the same issue with my 1970 MGB a few years ago. The rubber between the two halves of the harmonic balancer delamintaed, allowing the pulley to move relative to the half bolted to the crankshaft. It wasn't a constant slip, it was an intermittent walking slip that threw the timing marks off. Everytime I had to set the ignition timing, I had to find TDC and put a spot of paint on the crankshaft pulley.

I'm not sure if the Benz harmonic balancer is of the same design, but it might be worth taking a look.