Hmm yup sounds like over heated exh seats causes by unleaded gas. If the head have never been converted to run the unleaded gas. Or Very common problem. The unleaded additives does help. But it will not correct a burned seat or valve. But if you using the lowest octane gas this adds to the problem.
Burning seats start out on a heavy pull like your doing up a hill or with a camper on it trying to pass another vehicle and it take a nose-dive popping back out the carb. Also could be a combo of intake gasket bad and burned valves & seats.. Do a compression test first. Then no more guessing games. ...
orich..