OK getting the obvious out the way, make sure you are putting 450PSI into the reg.
Asuming you are doing this it sounds like your minireg has stopped regulating.
Remove the reg from the gun.
Then remove the circlip thats inside the top of the reg.
Now turn the reg upside down and carefully tap the innards out on a flat surface.
Chances are one of the seals on the reg is dead (you will be able to see it).