[Nothing But Foam]

I have recently just made a kegerator using the conversion kit from micromatic and a old refrigerator found off of craigslist. Heres my setup I have 6 ft. of 3/16 line, Beer in cup is around 40-41 degrees, my pressure is around 5 psi. Another thing that i didnt was set a cup of water in the refrigerator and after about a week there was about a inch of ice on top of the water, but the beer temp with a calibrated thermometer is aprox 40 deg. I have had a 1/2 barrel of Bush Light for about 2 weeks now and for the first 4 or 5 days it was pouring perfect and i was out of town this past week and when I get back I have a glass full of foam. If anyone has any suggestions they would be greatly appreciated.