why is my welding inconsistent?

You could use a backer made of copper or aluminum as a heat sink. If you are doing a butt weld copper works real good to bridge the gap and keeps the weld from burning through some. If it is a lap joint clamping a big chunk of aluminum of the side opposite of where you are welding works good to keep you temps down. Mig wire doesn't stick to alum (usually) or copper.