How old is the tree? If indeed it was planted in the 60's than it could be on its last legs, to check this drill a small hole, 1/4" about a foot from the base of the tree and no more than 4" deep, if the same comes out immediately, then its due to stress, if it takes more than 8 hours to see sap, the tree is dying, if you see no sap at all, the tree is dead, with only a few parts trying to survive, at this stage you might consider replacing the tree itself.
-Birdy