I faced this problem, and tried upping my memory limit, which fixed the problem for a time. But it came back asking for more memory!
That’s when I did more research and realized it wasn’t the memory that was the problem, but the fact that WordPress preloads the postmeta in the WordPress admin. So if you have a lot of custom fields associated with your posts, it’s going to take up a lot of memory.
I’ve written a blog post on what causes this, and how to fix this problem:
http://www.junaidbhura.com/wordpress-admin-fix-fatal-error-allowed-memory-size-error/
