Well, obviously there's an issue with the pressure regulator :-) (circuit that is). Could be a sensor issue, could be a wiring issue. With it only happening on a particular grade though the first thing I'd do is take a look at the wiring harness, perhaps at a particular angle the harness moves and breaks contact throwing the code and causing the problem, but on the level or different grades it goes away because it isn't at the right angle to make it happen. It's possible you have a pressure sensor that's going away, or you could in fact have a regulator that's beginning to fail. Start with the harness, make sure all the connectors are clean, routed and secured in their factory positions. Then go from there