Something is wrong. The reg should never have to be over 200psi, most run at 180. Dwell should be at 8ms with the stock bolt.
How far out was the backcap? It should only need to be about 1 turn out to get up to 300fps.
I'm guessing the poppet oring needs to be relubed or replaced. You might want to drop some oil in the asa and cucle the gun while you're at it.