Thanks everybody for the help,
I have finally resolved the problem, it was a shifter , it looked tight but when under the load it started to slip...It needed to be dismenteled cleaned a little bit and better tightened , now it works fine , i am very grtatefull.
Recently, a new local nice bike shop has been opened and guys helped me to solve the problem.
In the mean time i had a lot of work and didn't touch the bike , but fortunately i was driving my new one.
Thanks again ,
All the best , safe rides
Martin