Magnum and Wiring Swap

I was asking about fuel pressure because I know those GM filter / regulators operate pretty high (58 psi?) and the OBD1 Magnum spec is 35 - 45 psi. I don't know if your higher fuel pressure would cause the low RPM stumbling issue, but it's something to consider. Maybe it's pushing fuel past the injectors when the duty cycle is low and causing excess fuel to be dumped into the chamber?

FYI, I'm using a filter / regulator from a 2002 Jeep Cherokee. The spec for that regulator is 52 psi, and my car seems to handle it just fine.