so last night i drove my truck and when i started it, it died. when i started it again it it starting reving up to about 2 thousand rpms. i would put it in gear and its idle speed was just under 1000 rpms. but after i shut it off and start it again it would run normal. and when the trucks cold it will hesitate and if you step on the gas just a little too hard it will stall. but it only does it when its cold. but lately its done it when its running at normal operating temp. the only time it hasn't done it since i've owned the truck is when i had problems with the cat and i took the 02 sensor out so i could drive it to my friends shop. but today i took it out to see what it would do and the check engine lite didn't come on and it still hesitates. for a while i thought it was the muffler was so plugged up that it was backing the engine up. i bought the truck with the muffler on it and i don't know how long its been on there. i replaced the cat. and it has custom headers on it with an exhaust leak. so my only guess as to whats wrong with it is the air pump that sends air into the cat is messed up but idk where it is on the truck or anyway to diagnose it so any input on my problem is greatly appreciated