If the pipes are bluing and the gauges are bouncing then you running out of clean water. Had the same problem with my 35'. Are you sure the pressure doesn't go below 20 while running hard? The steps bring a lot of air under the hull. Do you have an air conditioner water pickup that might be causing extra aeration of the center drive?

If your drives are dual pick-up drives try plugging all the side holes of the center drive using a piece of duck tape. Use one piece of tape and let it wrap around the front of the drive so it doesn't blow off while running. See if that stops the bouncing pressure readings before plugging the holes permanently.