Before I give you any advice on this problem, one thing puzzles me; "you say the unit is 11 years old, and the duct, etc is too small." When did this "overheating on limit" start to occur. If it was because of the unit being too big and the ducts too small this would have happened right from the start 11 years ago..... An oversised furnace will have a "short cycle" effect, and will cause the unit to cycle for short, but oftener times.... (its best to have a unit the proper size, but slightly under is better than over.... Normally "limiting out" on a proper sized unit is caused by dirty filters, improper air speed, obstructions in the air flow, dampers closed,etc.
Again I ask ,when did this first occur.? Post back with this info and we will try to advise you further.Thanks. Hube