Some User Accounts Won't Remember Screen Resolution
So your question was asked ages ago, but before I was able to get my setup to work, I was searching for possible answers myself and happened across this page.

The key in my case was that an admin user was able to get resolution changes to stick, but non-admin "standard" users were not. Once I changed it with the admin user, all of the standard users would keep the resolution.

If that doesn't solve things for you, you mentioned trying a PRAM reset unsuccessfully. Have you tried resetting NVRAM, which newer Macs use rather than PRAM to store some display resolution info?