Hi everyone on board, it's my first post since ages.
i went on a month vacation and when I came back to drive my car it started some strange behaviour when engine is idle, after the engine is at op. ttemperature it dips as if it's gonna switch off then runs normal again.then runs smooth but emission is bad as I can see black smoke. Associated with this is strange dip been headlights the either turn on then off when I crank engine or they stay on and then my dashboard lights are always off and turning signals wouldn't work. When am driving sometimes the dash lights are back and I get a message in German "wasserwesen fullen"

Engine runs alright but I feel sometimes something dragging it back while cruising at 60 miles/ h with rumble sound it's so minor that u feel that the road is a bit rough.
any idea where do I need to look into? I feel that I got a shorted cable somewhere but I have no idea where to start

Car is e38 750iL highline Japanese specs 1998

Thanks in advance