Engine Mechanical problem
1989 Ford F150 5.0 302 fuel injected motor
Two Wheel Drive Automatic 145,00 miles

When going to the road whether I have the truck on cruise control or manually pushing the gas pedal after getting to a certain point the truck automatically starts to decelerate on its own, and to pick my speed back up I literaly have to push the gas pedal down to regain my speed, only in a few seconds for it to decelerate again. Would I have a passing gear problem, not really sure what this is though?

Thanks