If they are overheating, try cleaning out the roast chamber., Where the temperature thing presses against the metal, if there is crud caked on there from roasting, ie the char and baked on grease, try cleaning that off.
Also, if the fan is running slower because bushings are drying out etc it will overheat.
If nothing else changed and it's overheating the only thing it really can be is a reduced airflow, or faulty sensor for temperature cutting it sooner than it should. do you have an external thermometer you could shove an rtd down in there and see? if it's the fan you are kind of porked, because most of them are bushings not bearings and once they start to dry out, not a hell of a lot you can do. yah you can spray oil on it but it turns into a dirt magnet and it just gums up faster. Disassembly usually makes matters worse, unless you can get another fan or run two in tandem if th ey are getting that crappy.
Another thing to keep in mind, if the heating elements are wearing out, they are not putting enough heat on the beans, so you run it longer trying to get them done, that slow cooks everything, ie the thing is designed to withstand the heat of roasting / popping corn for say 5 minutes, but because the elements are weak,you are now running it for 10 minutes, the heat gets thru, soaks the walls and now it trips on hi temp because you heated EVERYTHING up trying to get the beans done because of insufficient heat being aplied to begin with.
Have you looked at the wattage it takes when you first got it /vs. now? try upping the heat a bit, I know it sounds crazy but give it a try, see if that pushes them thru till done before the high temp kicks you out.
Just a shot but might work.
Aaron
