Blank screen when I try to edit a newsletter

Hi,
I keep getting blank screens when I go to edit an existing newsletter. And other times the changes I make are gone when I go back in. My browser(Firefox) also froze when I was using the newsletter editor and when I reopened my browser not only were those changes obviously lost but also a newsletter I had previously deleted was back again.

This is a real pain because I wanted to set up newsletters to clone for future use as I was finding the template creation difficult.

Regards,
Nat.