If you place your mouse in the bottem right of a switchbar button, the tooltip will start flashing. The reason is because when you put your mouse on the tooltip it refreshes it but the tooltip allways has to be touching the button, so it refreshes but its still under your mouse, which makes it refresh again, and again, and so on.

No flashing on my tooltips.

mIRC 6.16, WinXP SP1

-genius_at_work

Nor here, mIRC 6.16, Windows XP Pro SP2

but it does happen on my 5.91 version running on Windows 3.11

Forgot: mIRC 6.16, Windows XP Pro SP 1

Also, im not saying you tested wrong but you have to put the mouse in the very bottem right, about a 4 pixel square in the bottem right.

EDIT: I cant believe i forgot to say in both posts that i was using a left aligned switchbar! Stupid me

Yep, confirmed here, bug is only with vertical switchbar on left side of window, in both XP and win2k skins. For all other switchbar positions, the tooltip moves off the button, for the left switchbar, the tooltip keeps appearing in the left bottom corner of the button, even if the mouse is there...
Ofcourse, you need a button that actually generates a tooltip, so use the buttons for status window or long channel names.

With this extra information regarding the alignment of the toolbar, I also find that I have that problem, although, due to the speed of my system, it's barely noticeable...just flashes almost too quickly for my eye to recognize.