2000 ford focus zx3 5speed

i have a 2000 ford focus zx3 5speed manual and sometimes the car works good but sometimes it will not rev past 4000rpm. sometimes it will sometimes it wont its sounds like the revlimiter but its not its a fast backfire when im at about 4000. when im sitting at the lights and ot has been doing it, it almost feels like its missing or not getting enough fuel and it will just die then ill start it and it will work like it should for about 5min. i have changed the spark plugs put on a new cold air intake changed the mass airflow sensor and it hasnt helped any help will be greatly apreceated.