the transmission upshifts mostly dependiant on mph.
while it may seem contadicitary it downshifts soley dependiant on the throttle rod conecting, but if its out side of the speed range it wont.
im not entirely shure about how vacume ties in with the equation but there are just two things you can do for the car that will get you right to the bare truth..
disasembel all vacume lines and run a lil less agressive cleaner than car cleaner through, say seafoam.
next have the fluid flused and filled, and replace the filter.
if the problem is consistant, internal workings are all thats left.
about the plastic gear, really not shure.
check out TempoTopaz.com :: Tempo Topaz Car Club of North America
for more insight