Welcome, I will do my best to help with your issue
If replacing the air vent the first time didn't help then the air vent is not the problem. Unless the water in the sight glass is over the top (boiler is flooded)then the feeder is not the problem. Assuming the boiler isn't flooded this type of problem is typically caused by the steam condensate not draing back to the boiler properly.
If this is a one pipe system this can be caused by a radiator that is not level or pitched back to the valve or it can be caused by a blocked return. If this is a 2 pipe system the steam trap is probably not opening consistently.
thank you, XXXXX XXXXX have a recommendation on a radiator plumber or where I can find an experience one? Thanks so much
I already tried Home advisor. Is there another recommendation?