Fuel pressure problems on a modded Z

So, after addressing my charging issue, I'm moving on to my fuel pressure.

I'm running a Walbro 255 inline pump that's on a Megasquirt relay with about 10ft of wire in between. Should be a pretty simple system.

With 12.5V across the battery at idle (14.1 @ 2000RPM), I'm getting 11V at the pump. So I wired in a hotwire from the positive battery terminal straight to the pump and got around 12.1V at idle, which is obviously better, but really weird considering I've got a direct connection from the battery to the accessory, so it should be the battery potential (12.5V).

So I decided to take grounding out of the equation by wiring another wire from the negative on the battery terminal to the negative side of the pump. Still got 12.1V. My fuel pressure evens out at 39psi, and then bounces around from 39 to 37 for about 2 seconds, then evens out again. It's really weird.

Should be rock solid at 40. Any ideas? Glaring obvious fixes?

The tank was dropped and lines were blown out about 2 months ago when this started happening. Everything was clean.

Has an Aeromotive FPR, 100 micron filter. JSK fuel rail, and running 440cc injectors. Lines run and return the same as a stock Z, just beefier lines.