OM615 1980Bj, 200D is tired, some questions

Hello dear W123 fans and helpers have now also switched from a W210/W124 to a W123. Currently I have a 200D with 393,000KM on the clock. Unfortunately, the otherwise reliable vehicle is no longer reliable. At the moment he starts very badly (only with full battery) when he slow organt I can get me wound organs until the doctor comes. With full battery when he turns faster he usually comes quite fast. When he is on then he runs first unround and wants to go out immediately. To accept gas he does quite badly, after half a minute the engine then runs round. But he does not do any pains with it. Further problem, if he then runs times, the tank must not be under half, or quarter full. Otherwise with full load e.g. he and the engine does not pull more, as if someone would pull the handbrake at once. If the tank is half full or above, the problem is gone. So I am currently tormented over. Have the tank sieve in suspicion, and diesel filter, can confirm it or something else.