2003 Chevy Tahoe starts hard

I have problems starting truck after it sits for a few days. If it don't start the first time you try you could spend five to ten minutes to get it started. This hapened a while ago could not get it started so I changed the spark plugs and that seam to help. But it is startting again and the spark plugs are far from needing replaced. It does seam though that it happen more when the gas is a quarter of a tank or less.
Thanks,
Mitchell