It sounds like you might have an air bubble in the cooling system if these things only started to happen after you changed out some of your coolant system components. Or, your t-stat may be installed backwards. I've heard it's easy to do with some MB engines.

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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.