car been running great now every time i come to a stop idle drops low and runs rough

I was driving along this evening and car was running great. Pulled into store and stopped it started running rough and idle dropped low so i thought i will advance distributor just to get home but no matter what way i turn distributor nothing idle dont go up or down. so jumped in and got on freeway to drive home it did ok ran rough but made it. it seems that it is mostly when coming to a stop