Have 97 montero sport 3.o. runs fine until it hits operating temp. Then rough idle. and backfire through intake. will catch up and run fine at higher rpm. Stumbles only at idle when you accelerate. Have replaced fuel pump, Mass air flow sensor, Idle control valve, stepper motor, EGR , plugs wires and coil pks. Still same problem. Throws no codes and no ck eng. light. Any suggestions.