Looking at Kerry's record from an independent voting analysis site, he doesn't seem inconsistent. (I highly recommend this site, too.)
http://www.issues2000.org/Senate/John_Kerry.htm
Anyone can take a 17-year voting record and pick it apart. There are complexities and nuances to particular votes and don't show up in a republican or democrat attack ad.
Remember, a senator is not the President, they share power with a lot of other people. I think Kerry's voting record is consistent with what he is proposing to do as President.