How do I erase a song I purchased on iTunes and reload it cause it cuts out

You should be able to delete it from your phone by swiping across it on the track selection screen (you should get a Delete button appear on it). Whether you can then re-download it will depend upon what country that you are in (music can't be re-downloaded in all countries) : re-downloading.

If you aren't in a country where you can re-download music or if it re-downloads in the same state then try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via Store > View My Account (Store > View My Apple ID on iTunes 11) and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find that song and use the 'Report a Problem' link and fill in details about the problem (iTunes support should reply within, I think, about 24 hours). This page, whilst not about how to report a problem, shows how to view your purchase history, and includes a screenshot which has the 'report a problem' button on.

Some people have had a problem with the 'report a problem' link (it's been taking people to this site on a browser instead of showing a form in iTunes) - if it does that to you then try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page.