I have a 96 12 valve with a nv4500. I can hit peaks of 1600 degrees Fahrenheit at 38 psi of boost And it’s still climbing. The egts take off like that in 4th and 5th gear after 2000 rpm when I’m on it. It has a slightly bigger turbo, slightly bigger injectors, a #6 fuel plate, boost elbow, 4 in turbo back exhaust and and a cold air intake I’ve been driving it and pushing it like that for a year now not holding it past 1200 for any longer than a couple seconds I just don’t like spending more time looking at the egt gauge than the road btw the egt probe is mounted on the exhaust manifold.  I’m thinking a different turbo or a better inter cooler or maybe a bit less fuel hope u guys can help oh and it dumbs black till 1650 and then cleans up to about 2300 and then goes black agin. Turbo has no signs of branding or anything and it’s the only upgrade that came on the truck that I didn’t do could it be a cheap/ inefficient turbo ?? 

Check for boost leaks first by pressurizing the system through turbo inlet, maybe start with 20 psi use soap water in spray bottle, look for bobbles. 

Might be the pump timing slipped more retarded the more the EGTs will rise.