I am also on site5 and having exactly the same issues with k2...

There is no solution for this issue, I just move my sites to another reseller. Sad but true.
For the new sites I'm creating and hosting at site5 I'm using the content component from Joomla, without K2, and it's working great for me, until Joomla 2.5 have a great improvement in this component. Also I have one site using Zoo component, and works great too, but still doing better with the Joomla Content Component.