Check all the hoses between the turbo/intercooler/intake manifold. Make sure you don't have a bad hose somewhere. You really need a boost gauge to see what's going on. And an pyrometer too just to make sure you're not overheating things on the exhaust side.
A boost gauge will set you back around $65 and is easy to install, no holes to drill. Just pull one of the intake manifold bolts and use the supplied fitting. You might check with a local diesel shop, they may have a temporary boost fitting and gauge that you could borrow. But I would check all the hoses and hose clamps first.