I did it
I think I've fixed my problem
I cleaned the distributor as shown in"FAQ Faults & Tips for factory Volvo Bifuel LPG / CNG" topic by CNGBiFuel and the most important I've fixed the stepper motor by re-soldering few points in the electronic, where I found problems! I think, that the main problem with the stepper motor comes with the electronic, because of the intense heat generated from the engine... because of the heat some of the soldering were broken...
It was relatively easy job
I drove more than 200 km without any issue. I really hope that my problem is fixed for long time ahead
A really BIG thanks to CNGBiFuel for the detailed information about cleaning the distributor!!!