Step 1 is to check your PHP error logs.

if you have an internal server error without take away any file, it will probably an a hosting problem, because of your server (normally shared) it's getting overload

On dreamhost, the same thing happened, what i did its to activate a crontrigger and the garbbagecollector, the problem suddenly dissapear. But i think it was because i submitted a tiket to my hosting company

Excuse my english, but i'm not from US