I recently bought an 89 xl ext cab, 302 with 4 speed teams, still working out some kinks. One of the major kinks is the transmission, it goes up to 35 mph no problem, but when it goes past 35 ish mph it starts revving high, if I keep applying the gas it revs high but goes up to 55, then it stops accelerating and just revs high. The thing is when I hit 35+ and let go then re apply the gas it just revs high with no acceleration. It won't accelerate again until the speedometer hits 25, then it will accelerate again. This causes a lot of cars to pass me up and cut me off since I'm constantly speeding up to about 45 mph then having to wait for my truck to slow to 25 before I can speed back up. Please any input or suggestions are helpful

 

I assume you are saying you have a 4 speed manual transmission? Your clutch disc is slipping, the only fix is to replace the clutch disc and related parts.

Does the rear seal on your engine leak?

 

I didn't read the title of your post(automatic transmission). I don't have an answer.