Boost problems ... Scratching my head

Little detail about the truck. Company driver first so I don't have maintenance history.
2006 KW T2000.
962,000 miles. Slated to be traded in on a new model next few months but until then my issue is. When I watch the boost gauge it only makes 18-20 psi. On rare occasions and randomly it will make 35-38 psi . I have checked for loose connections . Turbo boots , etc and visually I can not see anything wrong. I can "force" it into full boost sometimes if I get on the Jake brake then jump on the throttle. That will ramp it up for a short bit. Other times it will just randomly make full boost . I've brought the issue up to the mechanics and either they are not the brightest or don't care but I can tell even with out a boost gauge because the turbo will have a much different pitch in sound when it's pulling hard. The mecahnics said they pressurized the system and didn't see anything wrong, said they changed a sensor but can't tell me what. My guess is either just a bad turbo or a tired engine no making enough exhaust gas to spin the turbo fast as it needs to be.. That's all I can guess... Anyone else. Thank you

 

Cummins. It almost seams it's dependent on air / water temp . I would say bad turbo but it would never go into full boost. I also though waste gate is opening slightly letting pre boost out that I can spin it faster if I get on the jake then the throttle right away. Supposedly the company is trading it in sooner then later so there not gonna put a whole lot of effort into something that's not gonna be around too much longer

 

I did a little more research and it sounds more and more like a turbo actuator. Is there a way to do a quick test on one?

 

I have noticed if I shut the truck off for a while after things cool off it will regain boost for a few miles. Weird