odd behavior
 

I have a user form that contains multiple TextBoxes, ComboBoxes and
OptionButtons.
My annoying problem is that sometimes (I repeat sometimes) the TextBoxes
behave as if the "TabKeyBehavior" property is set to True. i.e when I press
the "Tab" button, the focus does not move to the next control, but leaves a
"Tab" space inside the active Textbox and I have to navigate through my
controls with the mouse.
The oddity comes from this behavior occurs about 10% of the times I use my
userform, and about 90% of the times it works normally as planned.
I have set the "TabKeyBehavior" property to "False" in the design mode, and
I have set it to "False" inside my code (at the UserForm_Activate event),
but this didn't forbid the odd behavior from occasional occurrence.
I Use office XP SP3 on windows XP SP1.

Thanks for any comments.