The downstream O2 sensor shouldn't cause surging, its purpose is only to check that the Cat is working. A used O2 sensor probably isn't worth the money, it could be duff. New sensors aren't very expensive if you shop around. I doubt if a resistor would fool the computer, the O2 sensor generates a voltage so you would need two resistors hooked up as a potential divider, they would have to be supplied from a stabilised voltage source (not battery). Its also quite likely that the ECU will measure the current in the heater circuit to detect heater failure and to ensure that the O2 sensor is up to temperature before it accepts any readings, you'd need another resistor for that as well.