Fluctuating Boost

I'm having an issue with my boost pressures under high boost demand scenarios. It's a Cummins ISX with a 10 speed. It's got no emissions equipment on it and has a conventional turbo, non VGT. When I first had the conventional turbo put on and the tune that came with it it ran extremely well. Had plenty of power, barely noticed a trailer behind me. Ran 43psi max. Heavy or light it pulled up a hill like a beast. About 2 months later there was significant power loss no matter weight or grade. Pulling away from a stoplight I can tell it's missing power. The turbo is acting strange when I'm pulling up a grade below 1300 RPM doesn't matter what gear, as soon as it falls below 1300 the turbo starts fluctuating about 5 psi. Back and forth rapidly. I can hear the turbo fluctuating with it. I never run up a hill WOT but for the purposes of recreating the issue I did. At the bottom of a hill pulling 30k lbs going about 68mph, 1450 rpm I put my foot down and don't see above 25psi. As soon as the weight slows me down enough for the rpm to fall below 1300 it begins to fluctuate between 28psi and 23psi. Back and forth rapidly. Serious power loss when this starts happening. Drop it down to 9th, RPM goes to about 1600 and it hits 30psi until I fall below 1300 rpm again it will fluctuate. Before this problem was happening, when I hit the hill at 1400 rpm and give it maybe 90% throttle it climbed to 43psi almost instantly. Now it never breaks 30psi no matter what and the turbo acts up. I had it pressure tested, found a tiny hole in a boot, replaced the boot, pressure tested again, held pressure. We thought we solved the issue. Today again pulling 14k lbs same scenario same problem absolutely no difference. Is my newish turbo already shot or do I have a bigger issue?