This for direct experience... Sounds like your alternator is slowing down. The belt is heating up and starting to slip. If the belt is getting old it will slip even if it tight. One sure test is to run it in Slow Down mode for a few minutes, turn off the engine then quickly touch the alternator sheave. If you leave some skin behind you will know that the slipping belt has heated up the sheave. (of course I am kidding about this test, but I am referring to Direct Experience ;o) )