Wish I had an answer but like you I get exactly the same thing happen on certain occaisions. You can usually get on the site with Refresh or the back button but since I changed to IE7 I tend to find that this happens maybe infrequently, but enough to be annoying, and also I have to keep re-entering my full bank security code details to get into my bank account rather than start at the normal password screen every now and again.
-------------------- I loved my grandmother very much.......and she fetched a good price on eBay.