I had the same thing and it turned out to be leaks around the flanges. Someone had replaced the studs with ones which had longer no thread zones. These worked fine for stock exhaust but did not allow me to tighten them enough to stop air leaks into the headers because the headers did not have as thick flanges as the OEM . If you did not have a problem before, then SS headers will not glow without some other problem. It's got to be an air leak. For insurance, put a washer on each stud, after the flange and then re-torque them to spec. Also look at the areas around the flanges and see if there is any exhaust blackening. If there is then that is the leak. Good luck.
Last edited by 911rudy; 02-07-2007 at 10:51 PM..