This issue is known as the "jumping effect".

This occurs when you have used an Anchor ID in your page in order to position your DHTML Menu relative to an Element but have not set the <Positioning> property in the AllWebMenus program to "Relative to an Element"; instead, it is set to "Relative to Window".

In this case, the menu will first show relative to the Window corner and as soon as the Anchor ID tag is read in the page it will reposition itself relative to that element.

In order to avoid this, just select the "Relative to an Element" Positioning, recompile and upload your updated menu.js file.