Omega B – Dropouts in rain/wet

In case of heavy rains (even after washing system with underfloor wash) and correspondingly wet road my Omega gives up the spirit (Omega B, Bj.98, 2.0, 16V, X20XEV). Sounds like 3 cylinders. No more proper acceleration and jerks when giving the gas. The instrument pointers spin for a short time. All needles dance up and down! Last time I was made friendly by the ADAC. When the morning started everything was good again. The master was himself but sure (100% motor electronics) that an additional mass band (motor body) can help. But did it not!! In the last rain the same thing, but then drove mercilessly and after approx. 200km everything away again. Maybe someone has a tip for me. THANK YOU! PS: Motronic lamp lights up – error code: Lambdasonde is broken. Does this perhaps have something to do with it?