Problem with my 2001 decelerating at 80 mph down to 70 mph and then sticking at 70mph on expressway.

I have a 2001 SR5 3.4L V6 automatic TRD off road in really great shape. I bought it in August 2015; it had one previous owner and he took really good care of it. It has 170k miles, the frame was replaced 2 years ago. I have put 4k miles on it so far.

I have noticed an intermittent problem and I was wondering if anyone has had some experience with it or might know what could be wrong. When I am driving on the expressway and I am accelerating past 70 mph to 80 mph, it will hold at 80 mph while i am still trying to accelerate and then slowly decelerate to 70 mph. I have found that either pressing the gas pedal down all the way or completely letting it up or doing one and then the other back and forth will eventually allow me to go over 80 mph. Most of the time I have no issues and it just flies past 80 mph without a problem.

I was hoping that there might be a solution that I would be able to do or that I could go to my mechanic with some idea of what is wrong.

Thanks!