I have entered crossing restricitons and speed restrictions to flight plan fixes in the Legs page, Executed them, and verified them only to have them change later on in flight.   Does not happen every time, but I believe if I make any other changes to the Legs page it wipes out all the speed/crossing restrictions that were entered previously.   Burned me once when I found the acraft not slowing down below FL100 and maintaining approx 300 kts while coupled to VNAV.  By the time I had corrected the issues, I was too high and hot for a stabalized approach and had to go around.   I am a little more cautious with it now and do not trust VNAV on descents anymore because even with it being monitored closely I have found the VNAV to porpoise on descent constantly adjusting descent rate from 500 fpm to 2500 fpm and back and forth again.   The airplane is flyable after 4.3, but is just barely flyable. Until VNAV is fixed, it has to descended via the other modes (Flight Level Change or Vertical Speed).