TRUCK DIES AT IDLE AND WHILE DRIVING

CHECK ENGINE LIGHT ON SINCE JAN. SCANNER SAYS BAD CATALYTIC CONVERTOR. DID NOT REPLACE IT. NOW TRUCK DIES WHEN SITTING AT AN IDLE. STARTS RIGHT BACK UP. WHEN DRIVING DOWN THE ROAD, RPM GAUGE SOMETIMES DROPS TO ZERO AND TRUCK SHUTTERS A LITTLE BIT, THEN PICKS BACK UP. FOUND A PACKRAT NEST IN THE AIRBOX. REPLACED AIR FILTER AND CLEANED THROTTLE BODY- CARBONED UP.