Wow something is seriously wrong there, whats with all the cpu usage spikes. Even if you ignore those 30% with nothing but firefox open is huge, I have similar specs and im getting 1-0% usage. I can't think of a forsure fix but what i would do is run memtest, check the temperates (you say they were those temps in the summer and such but check them now instead). If those failed I'd reinstall windows, and if that fails I think a new CPU would be the only option.