Well, I think it might just be a simply read\write error to the hard drive..
I did the following and it works for now.. if you havent tried so far:
stop all torrents
reboot (make sure utorrent does not automaticlly starts)
turn on the task manager
now start utorrent..
if it still hoggs all the cpu let it sdo so for few mins and see if there is a difference
if not than arrange the downloads by size and start them one by one, each one see if the cpu levels goes up and then come back to normal... dont start the next one untill the previous one finished.. if one is stuck for a really long time (specially if its not a really big one) then it might be currupted...
some of the files might need to be "tested" if the utorrent crashed last time and some new ones just needs some time to allocate space...
if the cpu goes up and then back down to normal after few min then everything is alright and you just downloaded a really big file that errored while allocating space...
I really hope it helps, it did for me and Im now waiting to see if it stays as it should..
good luck