With the engine running at moderately high RPM in neutral, no load, the intake manifold vacuum will drop after a few seconds, indicating an exhaust restriction. The engine is actually being loaded down like it was pulling a long hill just from the effort of pushing the exhaust gas out. The diagnosis procedure is pretty simple- - - -just create an exhaust leak by loosening a pipe flange, and repeat the no-load vacuum check. If the vacuum stays high with a leak, you can assume the restriction is further downstream. I've seen a catalytic converter empty itself into the muffler, and the no-power symptom remained after the cat was replaced. A new muffler cured the problem.
Jerry
