OS X :: Unexplained Clock Cycles / CPU Activity

I apologize if I missed a similar problem, but i think this is an unusual thing. Ok, so occasionally after my uMBP has been on a while (say, 1.5+ days?) sometimes when doing little more than doing a little web browsing, I'll see in both Activity Monitor and iStats that i'm using about 60-70% of my processor, but there is no visible process that is taking up the CPU. the issue is temporarily resolved by logging out or restarting. Does anyone have any ideas? I've tried running what available malware scanners there are for the mac, but nothing has come up.