i have a 2000 a6 avant quattro automatic transmission when it changes into 3rd gear the rpms drop a little stop and then drop a lot also there is a little shake in the engine any ideas?

 

well i ran into a problem over the weekend car stalled and started back up then got on the highway and car was smoking real bad so i pulled over and transmission fluid was leaking out so i had it towed to my mechanic and he said the transmission over heated and was coming out of the vent system so he tells me i need the transmission rebuilt and a new torque converter for $3200 i just want an opinion my car is a 2000 A6 avant quattro with 157000 miles had a lot of work done to it also