I would check the distributor caps and rotor arms.

The seal behind the distributor cap goes hard with age and allows moisture in, then carbon tracking and misfires start to happen.

If you remove the caps and see black lines between the center and the outer contacts then the caps (expensive) and probably the seals (cheap) will need replacing.

Good Luck

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1989 W124 300E
1991 R129 500SL