Your hardware specs look okay.
Do you have any non-internet connected games to use for performance testing? Are they slowing down too?
Run some hardware benchmarking programs, check temps on CPU/GPU to make sure they're not "throttling" during games.
Have you checked your internet connection? Visit the tools section of http://www.dslreports.com/tools
What background programs and processes do you still have running while gaming? http://alacritypc.kensalter.com/index.htm