00silverado09

Hello ive been having problems with my 4l60e ive gone threw 4 rebuilts i keep on loosing 3rd nd 4th gear once it passes 2nd gear it revs up like going on neutral.everytime i go to same mecanic(was under warranty) he says the sprag breaks n diffrent stuff everytime.i installed shiftkit maybe so it could help it and no still messes up.i was wondering if it could be the torque converter or i hav to uprgade to a 4l80e?thanks