I have a 2004. Corolla S with a 1.8. I bought it from a Toyota dealer with a 120,000 miles on it 8 months ago. It's now at 135,000. When car was in park, it would idle at 900rpm and drive 750rpm. It started running rough so, the plugs were changed, and it went smooth again for about a month and a half.. Now, it's running rough. It fluctuates when I put it in park to 1100rpm back down to 750/800rpm. In drive at idle its at 750rpm but, idles rough. I've checked the I.a.c valve, throttle body, air filter, and engine mounts. My fuel mileage went from approximately from 300 miles a half a tank to about 250 miles a half a tank when topped off . A/C wasn't used. I also fill up when I get to half a tank. Any insight would be greatly appreciated.