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
16V, I've installed the crank pulley at the same time.
I'll check if the EGR is well tightened and reset the computer. By the way, it might sounds dumb but reseting the computer just means unplugging the battery and repluging it , right ??!!