Disaster:Speed sensor,transmission problem???Please help
 

I have VW Jetta JLS 99 automatic transmission ,And I have this Problem:
When its on the drive "D" Gears dont shift automaticly, there is only 3-rd gear which works
car slowly accelerates,I can shift gears manualy 1-st then 2-nd 3-rd and then put it on "D" -drive ,still it wont shift to the 4-orth gear.
I went to VW autorized repairshop ,they check it on their computer ,computer said its "speed sensor" and they replaced it ,car worked GOOD for two months (it shifted perfectly),and then got this problem again. I went to another VW autorized repairshop they found again the speed sensor . It was good for 3 months this time.They advised me that its probobly Cage inside transmission wich probobly causing malfunction of the speed sensor.But they dont know for sure.
What is exactly wrong with my jetta ?Please HELP!!!