HD 6870 Problems

I have a Radeon HD 6870 (AMD) which worked just fine on a previous Windows 7 install. I upgraded the motherboard, which forced me to reinstall the OS as Windows 8.1. The new motherboard is an Asus Maximum VI Hero (Intel). Using the same card, the abilities of the card are seriously reduced, only able to play games at reduced settings and resolution. If I push the card as hard as I used to, it will crash the game or more often the entire machine. Any advice?

Upgrading to that motherboard should not have forced you to upgrade to Windows 8/8.1 there is a whole suite of drivers for that board on Windows 7.

However, I would go through and make sure you at least have ALL of the latest drivers for ALL of your equipment and not just the pre configured generic drivers Microsoft uses that are always months out of date or not even correct to begin with.