IF the charging voltage is constantly changing with the engine speed constant, then it is likely that your voltage regulator is not working correctly. If you can find some1 that will sell you a voltage regulator for your alternator you can take the alternator apart and change it. However, if you are having your mechanic do it the labor is prolly gonna cost the same as having a new alternator installed so it would be best to get a new alternator. When the alternator varies the charging voltage the electronics in the car can do alot of wierd stuff. For furthur reference, you would use a voltmeter or a multimeter to check the charging voltage. You can pick up a cheap one for like $15 at radio shack and definatly worth the money if u do your own diagnosis. I have a 20 year old meter that i keep in my tool box and it is pretty valueble when im doing electrical stuff. As far as the squealing when you downshift, your alternator belt might be worn and is slipping. Does it squeal for a sec or two when you start the car? When you downshift the belt is put under more pressure and if its worn it might not be able to hold on. For the problem with your subs, i would start by checking the charging system b/c it sounds like the subs work fine off the battery but do not work correctly off the alternator. Get a meter and check the voltage at the battery when the engine speed is not idle (pull on the throttle a bit). If the voltage goes under 13.5v (or over 15v) then there is a problem. Also check the voltage when the subs stop working. I'm assuming that the subs will stop working whenever the engine is not at idle, regardless if you are moving or not. I would also try searching to see if this problem has already been discussed or posting your problem in the Audio/Video section where the A/V guys can read it.