Consistently non-responsive programs, need to restart

I've been having this problem for months and it's driving me up the wall. I know for sure that it isn't a virus. First, my machine will be running smoothly; however, after about 45 min. - ~3 hours, programs will begin to stop responding; Skype will freeze, no matter what URL I type in my browser, it doesn't go anywhere, etc. Eventually, I'll have to restart my computer, and it's back to...