Pull the tap and hook up a regular pump tap. You should be able to pour on head pressure without pumping. See if it comes out foamy.
Also, the D-coupler should have a pressure relief valve. When you reduce the pressure at the regulator, make sure you blow the head pressure so that it equalizes at the reduced pressure.
You might have a bad regulator that’s creeping up in pressure.
My buddy has a kegerator and goes back and forth between commercial kegs and homebrew corny kegs. His setup has special fittings to easily swap between the two without changing beer and gas lines. He had problems with the commercial beers pouring foamy and after a few months he took it apart and found he had a gasket on one of the connections that was over compressed and deformed to the point it was causing a slight blockage at the fitting. He fixed that and has been fine. Just something that comes to mind…does he has a special setup like this perhaps. Any chance of a kinked beer line? That also comes to mind if it hasn’t been covered yet.
Oh yeah, this is something to look for too. Good call, Joe.
Along with cleaning all the lines and checking the regulator- you may want to check your gauges. I’ve jarred a gauge and thrown it out of calibration. It appeared to be working properly but was way off.
Burp the keg. If it’s pouring at 5psi it’s most likely under its own pressure. I bought a hand tap to get all my kegs exactly where I want them before I hook it to my tower taps. Maybe the beer was kegged early as was not done and finished in the kegerator.