I still have my old Sony PRS505 and PRS300, which I will probably sell of at some stage (or give the 505 to my mum) and have a Kindle 2i and DX (Kindle 3 on order).
The 505 was ok but the PRS300 did just what you are describing on a frequent occasions - something that drove me nuts and to try out the Kindle.
I never could quite determine whether it was some rogue EPUB formatting (one of the explanations offered) or the reader itself - I'm sure it was the reader as the same book would be fine on the PRS505 (though that threw a hissy fit once or twice).

neither of my Kindles has crashed so far and I have applied hacks to them and everything but they remain very stable.