Update: I mined stable for months after installing 7/32. Yesterday I retried to install 7/64 and the same story happened, no matter what driver or combination of them I used. Then I restored my 7/32 system backup and patched it for PAE (since 7/32 "sees" just 2.75 GB of my 4 GB Ram, and the patch allows it to use all the Ram available). Then guess what?  atikmpag.sys BSODs restarted. They happen even if I don't mine. So it must be something related to the use of the Ram.
I wonder if anyone else with 7/64 experienced anything like that and what drivers are they using.