It will help if you confirm which 235 you have and are working on please as the differences could help find your problem. You have asked for help a few times in the past to ID a few different 235's, from a 1952 out of a 5000/6000 Series Big Bolt to a couple of later 235's. One of those was a 1956 passenger car 235. Either way, you need to pull the valve cover and check your valve train for things out of adjustment/worn and even possibly a sticking valve.
Once you check it out, get back with some pictures of what you may suspect or find from your inspection.
