root menu items disappear (in 3 browsers/version)

[mev]

I have a strange phenomena: when I work a while in the cms-ms (latest version) of a site and click around several pages to test results, suddenly the menu items of the root diseapear after one other completely... just by successively pointing to them (even before any hovering is shown/visible)
The menu-items come back after refreshing the page, but shortly after this can happen again..

I used the vertical cssmenu {menu template='cssmenu.tpl'}, which is inside a

There are only root items, not any submenu's.

The disappearing happens in 3 browsers IE-6, IE-7 and Firefox 2.0.0.12 !!
It looks as if they vanish in a pair (2 items disapear at the same time, when pointing to one of them)

What could be the cause for this??
Anybody any clue??

Thanks much in advance!

Mev

[Mantlet]

I think it would be usefull if you posted some more info. Like a link to the site where this is happening and the code of the template and the css you are using...

Otherwise you probably won't get any suggestions or the first 25 will be completely wrong.

[mev]

Thanks, you're probably rigth!

It concerns this site:   www.rutgerschehrer.nl

This is the concerning part of the template:

{menu template='cssmenu.tpl'}


The css of the default vertical menu for CMS-MS, CSS Menu Module is attached,
as well as a css included in the template Andreas05, with a.o. this part:
#sidebar{
float:right;
width:110px;
padding-right:4px;
font:12px verdana,tahoma,sans-serif;
font-weight:bold;
}


My client now reports it especially happens after that they scrolled up & down on the scrollable pages ('Agenda' and 'Biografie"), the menu-items then suddenly move upards above the red border (#container)
which I can reproduce when I do that too...
So, may be it has to do with that css, which simply is:
#container
{overflow-y: auto;}

Don't understand this at all!!

Plse let me know if more info is needed!!

Thanks much in advance!

Mev