[v8volvo]

Those issues sound to me like you have a large restriction of airflow causing the problems. The black smoke / high EGT combined with lack of power suggest the fuel system may be just fine but there may be a serious inlet air restriction or exhaust restriction. The progressive falling boost pressure under heavy load and then eventual vacuum in the intake particularly sounds like a major inlet air restriction.

Can you give some more details/photos of what the intake and exhaust setup on your truck is? Do you still have the original Pinz snorkel setup installed for the air inlet? What kind of air filter and where?

What kind of turbo? What kind of intercooler and plumbing? CFM rating of the intercooler? Can you measure what the pressure drop is from the turbo outlet to the intake manifold under heavy load and high RPM? (Use multiple boost/vacuum gauges to measure this) How much vacuum exists at the turbo compressor inlet under full load/high RPM?

Any changes to the exhaust from stock, or damage like a crushed pipe or plugged muffler? Do you have the equipment to measure the turbine drive pressure (exhaust manifold pressure pre-turbo) and the backpressure after the turbo?