Ive had strange issues myself with graphics settings, in this case, sometimes out of nowhere, any dual render scopes would see a major frame rate drop, and no matter what settings I changed, I could not correct it, I had to use system restore to bring the settings back to working order.
On that note, all I can suggest, if its possible to use system restore to go back to when it worked, try that. If not, delete the settings file located at "C:\Program Files (x86)\Steam\SteamApps\common\Call of Duty Ghosts\players2\config_mp.cfg", and reinstall your video drivers, making sure to select "clean install" so all settings are reset.
See if that helps.
Additionally if none of those things work, try installing a older version of the video drivers, I had to do that to get mine to work.
Try using Windows Task Manager to make the game run at Below Normal priority once you have the game already running. Especially if you're using Sync Every Frame.
Also you can max out more settings if you make the game run at Below Normal or Low priority.
Other performance tips are at the Ghosts Optimizations Summary thread here: