Either a compression leak into the cooling system caused by a warped head... Exhaust gasses from a failed egr cooler.
Those are the two most common issues since it looks like yours is spewing I would say its more than a cap issue..
You'll need to hook a gauge up to the coolant hose that comes off the intake to the degas and read the pressures.... If you are on a hard acceleration and it is spiking then goes back down would be a egr if it goes up and stays up then a head issue.. Anything past 12 psi is no bueno
Thanks for all of your input guys, I really appreciate it. So far it's alive and well, no more leaking. I have plans to get it bulletproofed this summer once I start my big boy job after college, so I'm trying to baby it until then!
2004 6.0 Powerstroke
5" MBRP Turbo Back Straight Pipe Exhaust
SCT Livewire - IDP Extreme Street Tune