1995 Ford Bronco Harsh shifting on occasion

There appears to be an intermittent problem with my transmission. Often it works fine, but on occasion it seams to switch for days at a time into a mode where it slams into gear. When I shift into drive and while shifting later into higher gears it slams each one in. The problem goes away for weeks at a time and I dont see any pattern for this behavior. The car is completely stock and there has not been any changes made to cause this problem.