2000 Hyundai Sonata MPG down, rough idle

I've had problems over the past several months with my Sonata. Check Engine light came on, and I pulled codes for cylinder 3 misfire and system too lean. Throughout these months, the CE light will turn off randomly (it seems) while I'm driving, and then come back on a few days later, with the same codes. My gas mileage has gone from 28 to 22-23 and it idles rough when I'm sitting at lights, etc. I had just replaced the spark plug cords a month or so before this. I took the car in to get the fuel injectors cleaned, but that didn't really help. I cleaned the MAF sensor several weeks ago, which seemed to help a little. The light went off after this, and has stayed off, but performance is still bad. I had new spark plugs put in about 20,000 miles ago, and I put a K&N air filter in a couple months before all this started happening. Any ideas? I appreciate your help.