It's one of the most common problems with W126s. You have a worn out gear or gears in the speedometer cluster. They're made of plastic and wear out over 20-30 years. You'll have to replace the offending gear(s). It's a DIY operation if you know what you're doing.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.