Acheiva 92 Battery Problems + Speedometer

I have a 92 olds Acheiva and just recently it has been acting up power wise. First of all if im running enough power, Lights, Radio and Heat the battery Drains, I can see the voltmeter in the dash drain. Also now th Speedometers constantly Jump when idle. The Tac says im doing 8K RPM's and im going 120 miles per hour. im really confused. I got the batterytested and alternator tested at Advance AuTo Parts and said it was fine so im not sure whats up...when the speeometer jmps the engine isint reving or anything. To charge the battery i rev up the engine and sometimes that helps...Oh and the radio FM and XM make this realy loud Squeal that gains with Acceleration...I was told this is a grounding problem but I dont knwo whats up can somone help me!!!