With cast heads, a short timing curve, moderate compression, unknown cam timing, and 190-200 psi of cranking compression with the info you provided I’m gonna say it’s very possible that it’s detonating under load and when that happens the power goes away quick. What fuel do you run?
93 non ethonal honestly detonation that bad wouldn't show up in the plugs, metal particles, i believe I would hear it. But I still will back the timing for giggles.