Did you check wether all your downloads from Amazon has been completed?Go to home......menu.... view downloading items. That last one should be greyed out;else redownload that book from ManageMyKindle on your Amazonpage.
If there's something wrong with downloading; the Kindle keeps working away and away;that takes a lot of energy as well, and your Kindle will be unstable.
I have 1200 books on my Kindle and my Kindle isn't slow or doesn't crash.
Try calling Amazon again, as the quality of their technical assistance differs; though they are always friendly.