Hi Erikz, more then likely its your regulator if there is a electrical shop in your area that repairs generators / starters / alternators they can test the regulator to see if its working. I changed my old style vibrating points type Delco regulator to a Zeftronics solid state generator controller, one of the best things I've done it works great voltage is rock solid it eliminated all the static and noise in the radio virtually makes a generator work like an alternator the exception being that it will not charge at idle, and yes there is an STC it install the B&C light weight alternator on our planes I can't find a link for it right now.
A while back I posted it on the Champ site but someone must have taken offense and removed the link for what reason I don't know.
The Zeftronics generator control works great a new controler is only about $140.00 from Aircraft Spruce and that a lot less money than any alternator conversion would cost
http://www.aircraftspruce.com/catalog/eppages/gencontrolunit.php