Transmission (TC) problem

[edmsvx]

My son has a problem with his 92 SVX. With the Torque convertor lockedup all appears fine. When accelerating lightly the car starts to buck. If you accelerate a little more the transmission will unlock the torque convertor and all is fine. The transmission move very smoothly through the gears and the torque converter enages smoothly Any thoughs? Could this be a TPS sensor problem.

Another question.
How long should it take for transmission to warm up to allow the TC to lockup. My 96 will lockup if the engine is up to operating temp (basicaly by the time I hit the highway). My sons 92 take about 15 km of driving at highway speeds for it to lockup.