I guess there could be some driver problem, or bad update. If both occured, try to check if older drivers work better for you. Some updates may break fps, and i recommend you to report it as soon as you'll investigate the cause.
The problem is that both sli/crossfire never were a real gpu scaling technology, it was more of marketing thing. Hands down it helps with streaming and multi-monitor support, but besides that i have never seen any kind of rendering improvement so far