Problems With Fuel Pressure Gauge

I've been having problems with my fuel pressure gauge every since I installed it. The reading bounces around all the way from 0-100psi for no apparent reason. Sometimes its while I'm cruising, other times it's when I accelerate, sometimes it's when I let off the gas. Every now and then it seems to say steady at approximately the right pressure, right around 38psi. I checked the wiring and I'm pretty sure it's ok. I have a Zex wet nitrous kit so I put the electronic sending unit in between the fuel feed line and the NMU, which I assume should be fine.

I sent the manufacturer an e-mail and they didn't give me much to go on but said that the gauge wouldn't work on a return-less style fuel system. Has anyone heard of that before? It seems like the gauge should work no matter what type of system it's on. I'm thinking it might be the sending unit or the gauge itself. Anyone have any idea what might be wrong?