That machine runs for 2x7 for almost 2 years now.... It can run faster but for its application i need it to last longer. No need for new case the old case works just fine. In my situation The $3.99 solution work. It's worth trying before spending the money.
To be honest my first reaction to this post is change the case....But i remember i was on the same boat 2 years ago.
If it does not address the problem sure get a new case with 120mm fan.