Random freeze help please

[Jjames]

So the first samsung epic I had got bricked doing a fresh install of a new theme from a bad site i guess and it got traded in for a refurbished one. The refurbished one freezes all the time and i have to pull out the battery. its random and when it happens the display goes to sleep but the 4 buttons at the bottom stay lit up. any ideas on how to fix it? it would be appreciated.