RPM drop while warming up the engine...

Not sure,

  but sudden vibrations at low manifold pressure and power that goes away after a few minutes turned off sounds like carb icing to me. When you turn off the heat soak melts the ice and second start the engine is warmer.

Do you have an airbox temp sensor ?

Thank you Glenn,

It is not an ice issue.

I live in Israel. we don't have ice here?, more like tropical weather.

Maybe you have some debris/dirt in one of the carbs. Drop the carb bowls and check.

Thank you Garrett,

I just checked the carb bowls. they where clean.

I check it every 25 hours.

Nadav.

Mind you, Nadav,

carb icing is not a matter of ice cold outside temps (which you don't have in Israel). It may well occur when outside temps are up to 15 C (and more) (which is what you do have in Israel from time to time) when humidity is high.