Intermittent power loss

1989 Dodge Pickup - INTERMITTENT loss of power, especially under load. Acts like low fuel pressure but pressure test and flow test prove the "second" fuel pump to be okay. (It was replaced on a whim by another shop). Road test with gauge installed shows no drop in pressure. All sensor data looks great except the O2 sensor when this happens. It drops to.02v and holds there for a while. The O2 sensor has been replaced yet it still does it. It has a small exhaust leak at the manifold but it's on the other head. The idle is not excessively high and found no vacuum leaks. New tune up incl. Plugs, wires, cap, rotor, and the distributor has been replaced with a used yard unit. It's not just one cylinder going down, it's all across the board. Eveything just drops off. No backfires, no pops through the throttle body. Could this be a primary electrical problem as the newer Dodge trucks had with their charging systems? Any other ideas would be greatly appreciated.