Originally Posted by JimFreeh

That was my first thought as well.

Also, V belts can vary in cross section. The cross section needs to be correct for the belt/pulley interface.

Replacement belts often vary from the original specification in subtle ways not immediately apparent to a casual glance.

If the cross section is not correct, the belt can ride low or high on the sheave causing all sorts of problems. Generic replacement belts can also vary in length from original spec.

Replacement belts are sized to have the greatest number of applications using the lowest number of belts. Often, this means that the belt specc'ed for your car, will not be exactly what the factory installed. It may be slightly longer or shorter, the cross section may be larger/smaller.

Most of the time the difference is negligible, but there are occasions where the need for a factory sized belt is the only option.

I owned a 90 560SEL, and never had a single belt related problem. A good friend owned it prior to me for years, and he never experienced belt failure. He had belts replaced at the dealer, and I only source belts from a MB specific seller when I replace mine.

Thus, I suspect your problem is due to a specific issue with your car, either the alignment of the sheaves, or an inappropriate belt being used.

Addendum:

This is , of course, assuming that you have the correct tension on the belt. You do recheck them a couple of hundred miles after installation, Yes?

Jim