Problems solved
I was lucky when I took off the old old speedo and I took it to our local "small electronics Guru" for checking out if there is something to be done with it. He took the speedo apart and noticed that one condensator inside was leaking and that was the reason for the problems. He replaced it with new one. Then I ordered a new rubber reset button from wild steed workx (great service by the way
) to prevent water getting inside and now it seems the speedo is working like a charm
Thank you guys for help