Pinlock converted, Foaming issues..

I have a Smash in a converted pin lock and since it's been carbed, i've had this same problem. When it was first kegged and carbed I was still using picnic taps and opening the door, now however i have installed Perlicks and have completely switched over to 5 ' -3/16" beerline & 3/16" shanks. The problem has been the same all the way through. So here's the problem, when i either pushed the picnic tap or now pull the tap handle i get alittle bit of beer @ the correct pour and then it seems to ramp up and i get the rest it seems at a higher pressure or something. So i can get a 1/2 glass of beer and then the rest will be foam.

I have 2 other ball lock kegs that's haven't been a problem. I'm running 10 - 12psi @ the regulator into the fridge and into a distribution block. Beerlines are 3/16" and 5 ' long. the lines do dip down in the fridge and then back up the shanks on the door, i don't have the wrapped up or anything.

I'm not blaming the converted keg but at this point i'm kinda stumped.

If the pinlocks are sticking, then it will release the beer at a lower pressure and then ramp up like you describe as the pins slowly slide into a fully open position. This change in pressure is akin to opening a tap partially, and then fully.... it will foam due to the sudden change in pressure. I would disassemble the Qds on the keg and ensure they aren't gummed up... or bent... or otherwise being prevented from moving smoothly.

I had the exact same problem, but mine was due to ice buildup from uneven cooling.