2000 Buick Park Avenue RPM drop when holding steady speed

I have had this issue for a little while now. The auto would accelerate without any issues, but once i have reach a certain speed and mantain it for a few seconds the RPM drop by about 400. So i am holding the pedal steady to keep the auto at the speed, the RPMs drop by 400 or so and then if i press the pedal down the car is struggling to do so, then after a few seconds of struggling it starts to accelerate. If i am in the same situation, but this time i let go of the pedal a little and then press it again then the auto accelerates without any issues. This happens at any given speed it seems like, but only if i maintain one speed. While accelerating it does not happen.