It sounds like a vacuum leak, but there aren't any vac hoses that you should have to f*** with doing the header install. How long since you installed the headers and do you have a CEL? Did you do anything else to the car at the same time? Double check for any vac hoses damaged or loose, especially the one to the egr valve. Another thing I would suggest doing is to reset the computer, if you haven't done that already. I always do that after installing any significan mod and while not resetting the computer isn't likely in and of itself to cause stalling, is a good thing to do anyway. Good luck