Interesting update - hope you are using 91 or better gas.

Transmission SHOULD stay in the gear you put it in (L,2 or 3) until you hit the limiter - all ‘99s that I have driven do that. When it hits the limiter at 6000+rpms, it cuts out the motor intermittently to keep the revs down and save the engine. With or without the Interceptor. What year is yours? Please post the rpms and speeds that it ‘automatically’ shifts at in those three gears.