your torque converter is slipping for one of 3 reasons
1. the gearbox is getting the accelerate now message and unlocking it, this can be a throttle sense cable or switch prolem
2. The gearbox is not holding pressure to the lockup clutch, think strainer/oil hassles
3. the TC is slipping as the lockup friction plate is worn. Just like a normal clutch really..
the first 2 are relatively cheap to fix the 3rd not so...
