Could easily just be a bad cap. What are the last 2 numbers on it? If .00, know they changed the flange design on it from rounded to squared up which retains the pressure threshold better so the overflow bleed valve acts as it's supposed to (become the lowest pressure relief for the system at 19psi (IIRC).
I understand Porsche has a .03 revision available but at the least get the .01 part if you have the .00 currently (should be $15 or so - 996 106 447 01)
Old cap on left, new cap on right:
Threaded Mode
