I have 2 tips for you on VNAV descend, hope can makes you Enjoy the 757 before CS fixes it:
1.if you see T/D or descent path jumps wildly,check on LEGS if the speed near FAF is jumping, if so, manually input a speed restriction like 150 or 180 at or before FAF, it should limit if not stop the jump.
2. Just use FLCH when VNAV not working, guess what, VNAV wouldn't wokr perfretly IRL either and that's the way to deal it....will it's much better than CS's one for now for sure...