Open the developer tools (F12), start the script debugger and wait until an (uncatched) exception occurs, and keep an eye on the script console for errors.
Or: remove custom javascript includes and custom widgets until the project works properly again.
Test it under the 32 bit version of IE9 as well, I don't know if it is still the case, but on launch IE 9 64 bit still ran on the old javascript engine which was a lot slower / buggier that the 32 bit version.
Adding to Michel's answer, I suggest making sure all your widgets are up to date.
Are there any widgets used on the first page they see? Try removing these to test it if one of those is causing it.