What causes high RPMs at low mph?

Car shifts at 20 mph but RPMs are around 2000.  Car shifts at 35 mph but RPMs are between 3500 -4000 rpms.  Car used to shift at 30mph.  Letting up from accelerator allows car to shift sooner but it shifts a little harder.  Two tranny shops have told me that it's something w/ the engine that's causing it to shift later than 30 mph.  They have told me that the transmission itself is fine, all of the gears work fine.  It was suggested by two engine shops that it may be something inside the throttle body or the idle air control valve.  However, each shop had a different suggestion, not both suggestions.  The throttle positioning sensor has been replaced in Jan 2011.  At one point (after the tps was replaced) I was driving 55 mph & the RPMs dropped from 3500 to 2000 and back up again.