By 'slips', do you mean the RPM increases, but the car does not accelerate as it should ?
Or do you mean when it changes 1-2-3-4-5 that it slips between gear changes ?

If the former, then sounds like the torque converter.
If the latter, then sounds like a sticking solenoid.

In my experience, when a NAG1 is working 100% correctly, you can barely tell when it changes gears.