SL/R129: 1995 SL500 feels like going to stall
Have you had the heads pressure tested? It's possible that you have an ingress. The symptoms are similar to ones that I had a long time ago. Water in the cylinders will stop them from working smoothly. The 'rest' period is consistent with the water relaxing back into the sump. After which time you can start again, but the running will eventually get less and less, until either the head cracks, or one of the pistons bend.
First thing I would do is check the battery is the right spec (probably is), then get the heads pressure tested asap. If the heads are ok, get the timing checked, along with the plugs etc...



