Obviously, that shouldn't be happening with a quad core and the HD 6870. (btw, what resolution is your monitor?)
If you leave your PC on for extended periods, it is possible it is being attacked by malware, viruses, etc. And being loaded up with junk files, and registry errors. Be sure your virus and malware pgms are of good quality and updated regularly. I like Microsoft Securities Essentials and Malware Bytes.
I'd also run CCleaner and do the Clean and Registry sections to rid the system of reg errors and resource wasting crap.http://www.piriform.com/ccleaner