I have a problem with items not being selected when you use the keyboard to select them by choosing the first letter of the item's description.
You can replicate this on your demo site, so I won't bother posting any example code:
How to replicate: Select BMW using the mouse, then hit the "A" key for Audi, now quickly hit the "O" key for Opel. It should still be on Audi even though you are expecting Opel. If that didn't work just keep jumping around between items quickly using the first letter of their description, you'll eventually hit the problem.
The strange thing is, if you leave a few seconds gap between selecting the next item, things are fine.