I had a similar problem w/ my K1 and a nice new iMac. I found the following in the forums and it solved my problem. Maybe you should try it?
According to Scotty1024:
OK, I think I figured it out.
I've only hooked my Kindle 2 up via USB to a Mac Pro at home.
At Home I'm not running 10.4 or 10.5.
At Work I'm running 10.5.
I hooked up the Kindle 2 via USB to my Mac Pro at work and all hell broke loose.
The culprit is Spotlight. It is sucking down all your Kindle's bandwidth creating a Spotlight index of all the content on the Kindle. I'm researching how to club Spotlight over the head and keep it off the Kindle but for now it's definitely a problem (unless you aren't using 10.4 or 10.5).
UPDATE: Go to the Spotlight control panel's Privacy tab and drag the Kindle's icon from Finder into the panel. This will create a file on the Kindle that marks it as "Do No Spotlight This Device!" If you ever notice a directory on the Kindle named .Spotlight-V100 don't delete it! That's where the file is stored, in that directory.