If you're placing nodes every 90 degrees of the turn, the handles need to be perfectly straight to the pixel, in other words the orientation of the handles needs to be perfectly in incremeants of 90 degrees as well. The handle legnths inside the circle must be exactly the same, and be just the right legnth so it doesn't pump. Use the grid to help. Also, you can check for unsmoothness and pumping pretty easily by using the ride on track feature. (you don't have to activate the trains, just move around the track)