we have a poulan pro chainsaw. starts and runs great. the chain keeps popping off about 5 minutes into using it. it has been used very lightly over it's 3 year life, just to cut small diameter trees. last fall we put a new chain on it for cutting up limbs..everything was fine..now it won't keep the chain on, even though it's tensioned well and cuts perfectly for the first 5 minutes. any suggestions??