I had a similar issue once, for safety's sake I went with whichever gauge was reading the lowest, until I had the chance to use a different gauge and decide which one was off. Question, though. Did the gauge on your pump and the Marauder agree? The other option is to buy a new gauge for your fill station, they're not that expensive and are available at your local NAPA auto parts. BTW, I recommend NOT going with a liquid filled gauge, we don't need the damping, and I've had issues with them leaking.
Regarding the pressure dump, are you sure you're filling properly? You want to be filling slow, and bleeding fast. If you slowly open the bleed valve, sometimes the gun's fill valve can stay open, thus dumping pressure. This can be a good thing, say if you need to empty your rifle for shipping. Open the bleeder fast and wide and the gun valve should snap shut. If not, then you may have an issue with the gun valve, requiring pulling it and cleaning it. I believe the Marauder is using a Foster fitting with a check valve, it's not uncommon for a small piece of grit to get caught in one of those, of for the check valve to not seat properly. Good luck.
Dave