Reko, have you checked to assure you have a resistor plug?
Check the plug wire , is there any cracks or rub spots? Is the spark plug cap tight on the plug?
Is the ground wire tight.
I know you do not have any metal to metal connections, but do you have any metal that can vibrate against metal. I had a RF problem and it was a tail wheel assembly that drove me up the wall once.

Filters are good once you find the problem, but I would not rely on filters to eliminate the problem.
Good luck
Wayne