What controls torque converter lockup?

[NY 95 TSI AWD]

My torque converter will lock up in 3rd gear if I'm not on the throttle enough and then shift into 4th. In 4th it will lock up below 30 mph. I've replaced the tps and properly adjusted it. Also replaced the vss and tcu. What else controls lockup and what mph should it happen at? Should it ever lockup in 3rd? Maybe if od is off? Thanks, Keith

 

[NY 95 TSI AWD]

Also what mph should be the minimum 4th gear will happen? Mine will go into 4th at 20mph if I'm real easy on the throttle.