1987 Honda Accord Transmission Shifting

I drive a 1987 Accord 4dr with a 4 speed. Automatic transmission. It seems to shift normally up to 50 mph. Between 50 mph and 60 mph it upshifts and downshifts with the slightest acceleration or deceleration and doesn't stay in 4th gear. After 60 mph it stays in 4th gear. The throttle cable and transmission throttle cable appear to be working in sync. What could be causing this problem.I hope you can help. Thank you, Paul