i dont think its oil pressure related.........
mk1 suffered from a problem like this where oil would come out of the cap... vw solved this problem by putting a splash guard over the camshaft so when the cam is coated with oil at high rpm it wont spin off and hit the oil cap.....
when you take the oil cap off can you see the camshaft if you can your splash guard is missing!
and this will be your problem!
they may have missed it out when your head gasket was done!
this can also be caused by too much oil make sure the cars parked on level ground!

if you have the splash cover then the mesh inside the rocker cover may be blocked as someones already said. a vw garage told be they set fire to the mesh and burn the crap off or you could use a blow torch
if thats to be done id advise you wear a fume filtering mask as you will be burning oil........