When trying to maintain speed at 50mph or 70mph, it is impossible. I keep pressing on the gas pedal, but it continues to lag until I step on it hard to get it into passing gear. Then it will speed up and then slowly drop off again. It happens around 50mph and around 70mph. It was not happening today around 60mph. The lower gears shift fine. When the cruise control is on, it shifts all over the place like it is confused. Thought that maybe it was a clogged fuel injector. Put the cleaner in the gas tank and punched it a few time on the expressway. Didn't seem to help. Tranny fluid is fine. What are your thoughts and suggestions? Thanks!