Two things to do when switching video cards BEFORE you do the switch:
1) Uninstall any OC software you have (Afterburner, Trixx, etc) including any settings. You want to re-install it fresh so there's no conflicting settings and so that it detects your new card properly.
2) Uninstall old drivers completely, boot into safe mode, and run a driver cleaning program like DDU
If you've already installed the new card, that's okay. You will still follow the above two steps.
If you are still experiencing performance issues, but it is only a specific game, you can try changing that specific games application profile to "Maximum Performance" as has been stated earlier in this thread.
If you are STILL having issues, then you will definitely want to consider formatting and reinstalling the OS as has been suggested. I've never had to do something so drastic, but it doesn't surprise me that some people have had no choice.
It's important to keep in mind that even if it is the same brand that you've switched to, different architectures can still act oddly with old software/registry entries still present on the machine.
I've switched cards more than I'd care to admit in the past 4 years (including multi-gpu setups) and following the above routine has resulted in minimal difficulties experienced.
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