Surging over and over around 2000 RPM

My mechanic at the dealer said that it was probably the torque converter in the transmission and that it would be a 3000 dollar repair. He also stated that he would drive it like it is and see if it gets worse. If I speed up or slow down it will stop but when I get close to 2000 Rpm again the repeated surging starts again, this is very annoying. What do you think the problem could be, is it likley the transmission? Or could it be a cheaper less expensive fix I can try. What about a transmission additive? Would that help (if it is a transmission problem) Please help, me and my wife just bought a new house and are still trying to sell or rent the old one, have had other car repairs recently and other unexpected expenses so money is tight to say the least. Thank you