His truck is a '95. I would assume it's an A343F.I don't think my US-market A442F will drop out of lockup from throttle alone unless it's enough to trigger a downshift. It *definitely* doesn't drift in and out of lockup just driving down the freeway.
Tapping the brake pedal -- like you might do to disengage cruise control -- will cause it to drop out of lockup.
Have you tried driving while lifting up on the brake pedal with your left foot? (Obviously not in significant traffic)