had the similar on a PA38
When I was training, I had a similar problem on a Tomahawk ( normal carb stuff).
Engine was slowly losing power, selected hot air and it went very rough and i thought it was going to stop. Hot air off and power came back. I Put the hot air back on again but bottled out and turned it off after 5 mins and 1000' loss of altitude and limped back to Panshanger.
Problem was an air leak somewhere, though I don't recall where.
Hope it helps