It started about a month ago and has only done it maybe 5 -10 times since then. The car will start to shake and I notice the tachometer reads low, around 300-500 rpm ( it normally idles around 800rpm). Sometimes when i notice it I will take my foot off the brake to let it roll forward and this seems to prevent it from shutting off. It usually only shuts off when the engine is cold in the morning. It is a 1996 mercury sable and im in phoenix (even though its winter it rarely gets below 30's even at night). Also about a year ago my "check engine" light came on and i took it immediately to my mechanic and he said it's and "EGR" valve code. I asked if it needed to be replaced right away and he said "its only for emissions and and didnt need to be replaced until i needed to pass emissions and it shouldnt effect the cars performance. It's been about a year since then and I havent changed it yet. Is it that?