2001 elantra cutting out?

My elantra will run excellently, then bam! it loses power as if I left off the gas. I pump it and it will backfire then go again. It does not shut off however, just idles down. It has all, every sensor replaced! Everyone! New coilpak,plugs wires.charcoal canister good both purge valves good! New pcv valve. No leaks in intake, or head. Timing belt new. Fuel pump,and filter new. Runs great sometimes, so as mechanical crap does not go bad then fix itself off, and on it cannot be mechanical! It has been to three garages, they can't find anything! Cat converter good not clogged! Keeps putting up codes that say items already replaced are faulty. Service eng. Light not on. All wires, and plugs good, no problems with cranksensor, O2 sensors new! I think the main computer is going crazy, what else is there? Any ideas? If I get a new ecm, do I have to go to.a dealership to have it programmed, yes or no! Thanks!