Matthew it's Scott l read your problem now l ran into the same problem about 6years ago with a cat with a 4.3 gm tier 2 Spectrum system could not get spark at times. l know you said it has a reman ecm in it but some of them were not good they used resistor that were not automotive in side and would fail it would lead to no spark at times if you can l would send it to a company called flight systems and have it checked or get a new one from dealer. it was pot luck on reman ecms.