I agree it sounds like a trashy site.
Your "good target" sounds like a zinc-Lincoln, which would cause trashy signals at times depending on the condition of the coins plating.
I have a short amount of time on my 505, but it is a great detector, I pretty much followed Digger's advice from the start, or I too may have been lost.
Hunting in All Metal mode is key, you can still discriminate, with your ears and eyes, you will miss less then if you discriminate.
Most MDs work all-round better in AM mode. The X-Terras and others with TIDs and multi-tones lets you do the discriminating. You will have a better feel and confidence if you hunt this way.
Also a DD or smaller coil may be ideal for you, I prefer the feel and like the coverage of a DD vs a Concentric. I "feel" like my 6" DD covers as good as my 9" concentric, but with better separation. I'm not knocking the 9", it has it's place.
Sometimes it's just the site but you can be certain something is there, follow Digger's advice again, maybe slow down a bit, grab another coil or start digging the iron out.