bad alternator? or connection?, please help, its my DD Options

[cjh4l22]

ok so i have a 95st w/lots of miles but heres the deal... i've never had problems with it until tonight when i was driving it home. had the lights on and the cd player, everything seemed to be fine but when i would remove my foot from the accelerator to the cluth+brake the lights would dim and my battery power would drop from ~12-14V to 10. a couple times my cd player even turned off.. it seemed the harder i hit the brakes and if i used anything powered in the car without being in gear/on the gas the lights would dim. didn't notice the rpm's dropping particularly fast or anything... castrol syntec used in oil change 3k ago, new air filter, new fuses a few months ago, recently cleaned the batter terminals and had the battery tested.. its the darndest thing. thanks for any help you might have.