If I have the "Symbol" dialog box on the same display as the Word window, I need to put the mouse on top of another application window let it get focus, then move the cursor back to the "Symbol" dialog box and things work OK.
The bottom line seems to be the interaction between the Word window and the "Symbol" dialog box when x-mouse mode is set, but it you use another application window to take the focus from Word and then move the mouse to the "Symbol" dialog box to get focus things seem to work OK. I had set the "Activation delay" to 500 ms on my system (XP Home).
I really like the x-mouse focus feature of TweakUI, so hated to give it up and kept playing with the behaviur oddities until I say a pattern of behaviour. Let me know if this helps out your problem too. Good Luck...