Your symptoms would indicate a regulator seat oring issue or a weak regulator piston assembly. Since you are running an X-valve, I would expect that the regulator piston assembly is a high pressure version and it should be ok. They don't usually go bad, but it has happened. Even though you have changed the regulator seat oring, I would check it and the area around it including the brass piece and the valve and give everything a good cleaning. A piece of dirt could cause a bad sealing edge.
Lube the regulator piston assembly with the same synthetic light oil that you use to lubricate your mag valve with on a daily basis.
Except for the Automag in front, its usually the man behind the equipment that counts.