Sputter & Knock Problem
 

Recently my car (bought 2 months ago - 2004 ATX w/50K miles) has been sputtering and almost stall when sitting at a stop for more than ~30 seconds. Also, when I start from a stop I can hear a little clatter like a spark knock. I run 87 octane and have only recently had this happen. Any advice or idea why this is happening?