You might have a vacuum leak at one of the evap hoses which is allowing too much extra air in at idle. Also, check the rubber intake elbow for rips or tears where air can get in past the MAF. Check to see if you can suck or blow air through the evap line up near the intake. But be careful, you might get a mouth full of dried out charcoal if the cannister is going bad. Don't ask me how I know!
