20 degrees does sound a little high, though as said, ADF's do wander somewhat, particularly at certain times of the day. Comparing the ADF to a GPS is really comparin technologies of two different eras. ADF's have their place, but don't expect the performance of a GPS form an ADF. Yes, you could have it serived, but ask yourself, Is it worth it? You could spend the value of a GPS, brining the ADF back up to the perfect specs. Most people I know with VFR aircraft are not investing in their ADF's very much these days.
Pilot DAR