It appears I am having the same problem with mine:
click here
The up-shifts are observable for 1-2 (@15 MPH), and 2-3 (@ 25 MPH); 3-4 is murky at best, very hard to observe, with very little drop in RPM - then comes a big rpm drop at 45 MPH, presumably when the TC finally locks up. Slushbox indeed! But it's the TCM that controls the transaxle and makes it into slush!
My car is practically new, and as in your case, my wife drives it 99% of the time. I can't afford attempting tinkering with anything that might void the warranty.