tranny problems

i took my svx to the dealer today because i couldnt get it to shift normal..well they said before looking at it , its probibly the tranny..but what im wondering, does the tranny just go out in one day, compleatly? i mean, in the morning when its still cold, it shifts normal, no problems..but then after about 30 mins, i hear a buzzing sound, and the shifting gets a little worse, and then is unable to hold a gear at all. i can shift between all gears, fwd and back, and i feel no change..but if i shut the car off for just a second, and restart, it will be ok for a few mins.also the fluid isnt burnt at all...i was wondering if the tranny oil pump went bad, looks like it would do the same thing.