I have a 1996 GMC K2500 Sierra 4X4 HD with 119,000 miles (5.7L, automatic trans). The transmission fluid is clean and smells normal. It was changed about 30,000 miles ago, but the old fluid was OK as well. I have NO problems shifting or pulling my boat (18' maxim). The problem occurs when I am coasting down hills. The transmission will not stay in gear when the speed hits 60mph or above. It does not matter whether I am in OD or 3rd gear. When the wheel rotation exceeds the rpm, the transmission goes to Neutral, the rpms drop to idle (~1000 rpm). If I rev the engine, it will kick back into gear. If I keep load on the transmission it will stay in gear, but when I let up on the gas it drops back into N. CAN SOME ONE PLEASE HELP ME WITH THIS ANNOYING PROBLEM.