380SL 1981 erratic idleing on 1/2 tank

[AlexK]

Good Day all,
My 380sl 1981 has a nasty problem:
-When the gas tank is more than half full than the engine runs fine.
-When the tank is less than half full than the engine runs erratic, from 500 RPM to 2000.
Finally shuts down, and than a gurgling sound comes from the tank like it is sucking air back in. After that the engine is very hard to start. The problem goes away when I add 40 liters (the tank is that more than half full). The previous owner already knew this problem.

I figure it must be a dirty filter inside the gas tank, if there is one. Things are fine when the filter is completely submerged, after that the trouble starts.

Any help would be appreciated.