i'm at my wits end with my 1997 nissan altima. i have an intermittent problem. it runs great when i first start it then after awhile it starts chugging really bad and backfiring then eventually it will stall and won't start. let it cool off for awhile then it runs fine for awhile again. i have replaced the fuel pump, fuel filter, fuel pressure regulator, coil, battery, maf sensor, replaced catalytic converter with a header. did a routine tune-up about 8000 miles ago which includes cap, rotor, plugs and wires. if anyone has any ideas let me know please i don't know what to do.