I have a 02monte ss and it's been back and 4th to the trans shop since january when i had it rebuilt...for some reason when i go past 45 the car starts to shake like it's falling apart..i took it to the dealer and they said that it's the torgue converter jamming up..can someone tell me why it keeps doing that?

Because a lot of trans shops that don't know the 4t65e very well usually screw stuff up.