I had a situation where one of the pistons in the injector pump
was sticking. The engine
was pumping out clouds of dry, powdery soot which was literally piling up. If I cracked one pressure pipe, it would stop, then restart as I went past a certain rev threshold. Rinse, repeat. I cured it by taking the cap off that piston and repeatedly tapping it down with a hammer and brass rod. It is more likely being cause by a constriction in the air flow though. Try running without an open inlet manifold.