2008 Chevrolet Silverado Check Engine Light

My check engine light came on. I checked the codes they were PO442, PO449, and PO455. So I changed out the gas cap. Then I changed out the Purge Valve Solenoid. After changing out the Purge Valve Solenoid my light went out for about 5 days, then it came back on again. it stayed on for about 4 days, then went off again for about 2 days. Now it will stay on for about 2 weeks, then it will go off for a day or so. I don't want to just keep changing out parts, I would really like to find out if there is another reason that it would keep going off and then back on. Here were I live we have to take our vehicles through an emissions test every year to be able to renew our tags, so I really need to get this fixed.

Thanks