P0299 code + APR tune + REV D Diverter=PLZ Help!

2007 GTI. I just got this car. Bought it with a CEL for underboost. The car had been previously modded and has a BSH intake, BSH PCV plate, heat shield and BSH boost tap. I've got an aftermarket boost gauge as well and have been only getting a solid 4 PSI...the boost builds very fast but stops just shy of 5 psi. At first I though that it was a DV, so I crawled up under there yesterday and took off the old one...I found an updated DV with a GoFastBits spacer plate.

I do not hear any boost leaks at all. I figured maybe it had something to do with the APR tune, so I got in the car and set the second (2x engine light flashing) option as it was previously on 3 flashes. I had boost! I didn't get close to flooring it, but I had over 12 PSI easy. It pulled like a freight train! After driving in steady interstate traffic for 10 minutes, I went to pass someone and had no boost again. :/ I pulled over and selected the first option in the tune, which should be stock and still no boost.

Could this be a boost control valve? Any advise would be greatly appreciated!

Sounds like you still have a leak somewhere. Check n249 hoses, charge pipes, PCV

Best bet is that if you do not see anything obvious which would cause the leak, is to smoke test it. From the turbo inlet to the valves. Any leak will be discovered if you do this. And if you actually have more than one and were only to find one with a visual inspection....the smoke test will have found them all. N75 can fail by cracking about its three hose ports or failing electrically. So with it, you'd need to remove it to visually inspect it for cracking or try wiggling those ports to check its solidity...and you might as well replace it if having gone through all it takes to remove it... and then, testing it electrically you need to unplug it and test the resistance across its two prongs. IIRC there should be 25 to 35 ohm and if it is slightly out, you'd replace the N75.

I’m local (south side Indy) if you need help troubleshooting