97 grandprix problem

When engaging car in either drive or reverse a very strong electical outage happens for a brief second, the lights dim, brakes fade. i had my alternator checked and it was fine, battery is newer and in good shape....any ideas i thought the voltage regulator was bad however it is internal in the alternator and would have showed up on the test...any help would be great..thanks.

Check your electrical grounds especially the ones that ground to the engine. When the engine rocks it is causing the ground to move and it is losing brief contact causing the lights to dim. It is also possible that the positive could be loose at the starter so check that too. Good contact is essential and anything that increases resistance will cause the lights to dim. The first place to start is the battery and work your way from there. check all connections and you should remedy the problem. Hope this helps.