Troubleshooting - Lean under boost

[88turbobird]

Got lucky with some good weather in NJ this past weekend so I had the bird out. The car ran perfectly all day on Saturday. Go to drive on Sunday and for whatever reason, it's going lean at anything over 5 psi. Some details about the car:

88 TC, pimpx, 60lb injectors, GT35R turbo, kirban adjustable FPR, unknown fuel pump (owned the car for about 4 years now). The car idles and drives perfectly fine when not in boost. AFR's look great during cruising, load (no boost), etc. The AFR's start to drop as I get into boost but anything over 5 psi and it starts to go lean. I'm thinking it's a fuel delivery related issue. I haven't had a chance to take a log and dive into it just yet but I wanted to get some ideas to start troubleshooting. I'm thinking it could be one of the following:

Kirban FPR is failing (was on the car when I bought it; unknown age)

Failing/clogged fuel pump

Any other thoughts on this?

[Stinger]

I'd suspect the pump/filter but you won't know for sure until you put a gauge on it that you can view while driving. I've used the diagnostic kits with the long rubber hose and zip tied the gauge to the wiper or taped it to the windshield before for this purpose.