When Category is selected Search Form shows field box momentarily

[Chrisj]

I have been working on a Search Form, that has a small issue.
In the Form table, when a Category is selected, the Form shifts momentarily.
What is taking place is the Sub-Category table section momentarily 
shows a small sub-category field box, and then disappears (which shifts the Form).
I'm trying to remedy that issue, so that the Form doesn't shift, and the 
small sub-category field box doesn't appear.

 

For a visual, I've attached an image showing a series of images,
the top image(1st) is before a Category selection, the 2nd shows the Category drop-down, the 3rd shows what happens upon selection (the field box shows), the 4th shows the that the field box disappears. (this may be a jQuery issue - not sure)

Any ideas will be greatly appreciated.