Help with selecting from drop-down list

I have a form with a drop-down selection and have a couple of small keyboard data entry queries:

  1. I used to type enough characters in the Add/Search box to narrow down the selection before tabbing down to the required value and hitting enter. But tabbing doesn’t navigate though the list any more - is there any way to get this back? Also I now have to start off by tabbing into the Add/Search box whereas I’m pretty sure the cursor used to start there automatically which was much neater.
  2. I’ve long had a problem that I would type in the Add/Search box and see the selection narrow down. But often the first few characters then suddenly disappear and the selection widens up to match the remaining search string. I can’t see any pattern to when this happens.
    I should mention that the drop-down is selecting a key field from the same table but everything else works fine.