I currently own a Saab 1998 900 SE Turbo with an idling problem. After a few minutes of idling, the engine seems to "chug" every 15-20 seconds or so. Every once in a while, the entire car actually stall out. I have replaced the IAC valve and PCV nipple, as well as checked for vacuum leaks using carb cleaner, but the problem persists. Any ideas on what the problem is? Any insight would be much appreciated.