Cat that keeps getting swollen mouth????  

My 4 year old cat goes through periods of where his bottom lip will swell. I thought about what could be causing this but it has done it for a long time now. It will get better then a month or so later it gets swollen again. I've thought of everything from biting on paper to rubbing it on the screen looking out the window. He is a house cat and has had his claws removed as well as neutered. I've had him on a healthy diet since a kitten and even had to put him on a Science Diet formula due to his size.
I've moved residences with him and it puffed up at the old place too. Any ideas as to why his lip and chin could keep puffing up from month to month even though he don't bite on anything other than food?