2004 Hyundai Elantra Transmission problems.

Bought car at an auction. It had a severe clunk when going into 3rd gear. Serviced the tranny fluid and clunk was still there, not when you first turned on the motor and drove but shortly after the engine warmed up. So he changed both sensors. It seemed to have gotten worse after this, so he changed the ECU with a used one. It seems to do okay when I drive the car hard. No clunk when I drive fast. I had been driving the car slowly and I manually changed the gears trying to avoid the clunking and the jerk of the car prior to the ECU. It didn't fix the problem so I tried something different I pressed on the gas and gave it more rev and it seems like its running better this way. I know its not entirely fixed, its just a way of avoiding the clunk and the jerk of the car. Have any ideas on what may be going on with my car. In need of a final repair. Thank you for your time and your help on this matter. Oh and just so you know my air conditioner stopped working after they put in the ECU. Also we have no engine light on, hope this helps.