I have had this happen on high mileage/dirty 235/216s. I have always suspected a jammed/gummed up lifter. Pull the lifter if you can. Hopefully the lifter is not too dirty to pull it out of the bore. Clean the lifter along with its bore. That will usually take care of it until another piece of crud jambs another lifter. Fortunately, it never seems to damage a valve or piston, usually only the push rod gets bent.
Many of the vendors (Patrick’s antique cars and trucks) listed on this website can help you with the parts, if you cannot turn one up locally.
