Did you have Limewire open at the time? Best not to for accurate results. I wonder if it's due to inconsistent line between you & the ISP, etc.
Your inconsistent speeds is a part of your normal use of the internet which is odd. I'm thinking this is either an isp or line problem.

BTW the initial figures you gave should be kbits/s not kbytes. The 2nd figure in brackets is KBytes.