Problem with a Tlistbox using an external TScollBar

I use a TListBox that never has more items in it than it can display. When
the user attemps to walk in the box, it changes the contents of it's self to
simulate a scroll (I dont want to fill 200,000 items in a Listbox). This
works very
fine when the user uses keyboard (arrows, pgup, pgdn) I also capture the
mouse
to know when it should scroll up and down.

Now to the problem. I implemented an external Scrollbar (as the TListBox
never
shows a ScrollBar as its never full). I hook the scrollbars onscrollevent
and everything
works fine (the ListBox actually scroll) BUT BUT, sometimes the scrollbar
gets messed up
it still works but it seams to lost control over how it repaint itself. And
sometimes, if I
press the mouse on the scrollbars uparrow. Holds it for a few seconds, and
then release
the mousebutton, walks away from the scrollbar with the mouse. And later
goes back to the
scrollers uparrow, it click it by itself.It acts like a button does if you
have the mouse on it and
press leftbutton, walks away from the button and then back again but in my
case I have
release the leftbutton

Why is this happening?

LB Cid