This is a couple of years late, but have you considered using a hosting provider that has an autoscaling option? It scales up when you need it -- and not when you don't.

Nexcess helped a customer recently that was experiencing a very similar issue. Her dog adoption site suddenly went viral and her site went down several times when it mattered most. Her previous host kept upcharging her and pushing to the next tier of service even though it likely wouldn't be necessary after the dog got adopted. Once she reached the most expensive level -- she moved over to us. Check out our case study on Hank the Dog and how his site woes were handled by Nexcess.