Our 2007 Element has been idling rough for several months. The CEL light just came on and we took it to our mechanic. His computer identified cylinders 1 & 3 aren't firing right. It only seems rough while just standing still although there might be some decreased acceleration power. He's about to do a compression test. He asked us to do some looking on the web as it could be so many different things. The fuel trim is good. Vehicle has 118,000 miles on it and has had scheduled maintenance, oil changes, transmission flushes, spark plugs changed, etc. We have not had the valve clearances inspected as recommended at 110,000. Any ideas?