Kegerator Beer Line Woes

Hey all,
I recently acquired a run down kegerator and fixed her up to be a show piece (in my mind). The of the taps work flawlessly, however the fourth is being stubborn. For some reason the beer doesn't want to come out how it is supposed to. Instead, it pours foam and immediately after closing the faucet, the beer in the line begins turning into foam with air pockets. Any idea why this may be? I tried replacing the beer dip tube o-ring to no avail. It is holding pressure just fine. The only think I can think of is that it is over carbonated, as when I opened the lid all there was was a dense head of foam which was different from my other three kegs. Any and all help is appreciated!
-Shawn

Try a different keg on that tap, that will tell you if its the tap or the keg