menus are sometimes unresponsive

Binary package hint: gnome-panel

When I hover the mouse cursor over a sub menu, normally the sub-menu opens very quickly, in a split second.
However, sometimes it hangs for over second until it decides it's willing to open the sub menu.
It happens randomly, but it happens all the time, that's why it's annoying...

I attached a little video (animated gif) that illustrates the problem.

Computer is plenty fast enough (hell, even a pocket calculator ought to be powerful enough to draw a menu...), system is idle/unloaded, and since the menus have been already read/cached a hundred times (since I never power-off or log-out), and I have bags of RAM, all the icons are obviously cached so there is really no reason to access the hard disk to draw the menu, so it ought to be instant all the time.

I first noticed this bug 4 years ago exactly.
4 year later, I have gone through all Ubuntu versions, changed hardware several times (motherboard, graphics card model and brand/driver) and it is still present. It even shows in virtual machines !
I vividly recall that Ubuntu 4.10 did not have this problem. I am pretty sure it appeared with Ubuntu 5.04, when gnome replaced the two menu setup (Applications/System) with the current three menu one (Applications/Places/System). Maybe the change in the menu structure back then, brought some new code that causes this bug...

I am using Ubuntu 8.10 with all update, but as I said any Ubuntu version bar the very first one, exhibit the problem.