Diesel Bombers - 6.5 Turbo Problems?

Hello everyone, I'm GMCforLife. As my name suggests, I really like GMC, as I've grown up with them my whole life. My grandparents would always take us camping, and grandpa would use his 1994 GMC 3500 dually 6.5 Turbo to get us there. Needless to say, I'm in love with the truck, and still drive it to this day.

A little bit about me, I have a general interest in engines, and diesels in particular. Nothing like that diesel smell! Anyways, I don't own the truck, but there is a problem with it. The turbo seems to be going out. What exactly happens is, sometimes when I start the truck and begin driving, I hear the turbo fine, and the boost gauge reads fine, and I have normal performance. But, sometimes it will drop out of nowhere, and I can still hear the turbo, but now it sounds more airy, is the best way I can describe it. (The performance doesn't seem to suffer, but I'm not really pulling anything anyways) I'll accelerate, and the turbo will sound, but just more like air. Also the boost gauge doesn't read during this time, unless I give it a bit, then it barely reads.

Also a note, not sure if this is important or not, but I want to include every detail possible. When driving on the highway, I'll approach 65-70 and the check engine light will come on. If I accelerate past about 70, it goes away, and if I slow down below 65, it goes away as well. Not sure if this is related to the turbo.

Either way, thank you for the read, and I'm very excited to hear your ideas. I'm very new to the diesel world in terms of terminology, so if someone could point me to a great source they have where I can learn the terms/concepts, that would be very appreciated!