97 expedition 4wd eddie bauer edition. 5.4 2v v8 250xxx miles

Long story short, voltage gauge drops to 8/9v when i hit the brakes or use turn signals. When this happens i lose power to my interior lights, brake lights, radio and cluster dims. Alternator seems to be good, ran the truck without the battery and it did not stall. Battery tested fine as well. Problem was intermittent up untill last week. Cant remove the alternator at the moment to bench test it as its my daily, any advice?