1986 Ford F150, automatic,2WD, works fine the first 5 minutes of driving then it does not run right.?

It works fine. shifts up all the way to 65mph ++ but then about five minutes in operation it slows down to 20mph and does not want to shift to third or upward again. I accelerate but sounds like it wants to stop so I ease off and go 20mph back home. Already check the transmission oil (in park- idle while engine hot) and it was good. Siphoned some out and replace it with Lucas Trans. fluid but no improvement. Same deal. Added Lucas fuel additive also but same thing is happening. Anyone know what the problem is here please help. thanks