Update 7/28 - A new client has been made which fixes this issue. This client was released late Friday night. It was released as a required patch. If you do not automatically download the patch, please download the full client release from here:
http://panther.there.com/download/thereinc/prod/download/Install_There_20080725.signed.exe
You cannot log on without this version of the client. If you can log on and see your avatar, then you already have the update installed, and you don't have to worry about blinking menus anymore.
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We have determined that there is an issue with the latest update where your toolbar menu will flicker and reset when an avatar appears on your screen.
This may also be affected by avatars appearing even when they are behind your avatar.
In crowded places such as Paiz or Zona or Zephyr, this can happen very often as avatars move in and out of the "20 closest avatars" visibility range.
We are working on determining the cause currently.
-There Customer Support