A few troubleshooting things that come to mind.
1. How long are your beer lines? I believe that about 10 feet is optimal for less foaming (Mine are about 6 feet and I don't have foaming issues)
2. What psi are you serving at? About 5 psi should be sufficient, if you are higher then you're putting more CO2 into solution in the kegs through force carbonation which could lead to foaming. Did you release pressure from the kegs when you turned down the psi?
3. It is possible that the keg has froze, I had issues with this when I built mine and the kegs were touching the cold walls (keezer).
I hope this helps.