Grumpy_Mike:
You seem to be remarkably reluctant to accept advice once you have asked for it.I do not accept suggestions that make no sense for me. Suggestion to use diode is not feasible in my case with AC relay.
Interference can also be airborne as well as conducted. I am not sure what else we can tell you. You have an interference problem and unless you treat it at the source then your project stands little chance that it will work.
I agree and I think that in my case the problem is generated by radio frequency ( because circuits are separated).
So the good suggestion is to use snubbers and that is what I will try.
And with this thanks to all nice guys that gave me ideas.