Qt 4.6 in Windows XP -- Tooltips Sometimes Appear with No Text -- Why? How to fix?

[CSeggelin]

Tooltips are not rendering properly in Win XP (sometimes). I'll hover over a button, and I will see the appropriate tooltip, then a moment later I will hover over it again and a blank tooltip will come up. The tooltip continues to be wide enough to display all the text that is in it, although its height seems to have reduced by 50% and there doesn't appear to be any text in it. I am using a stylesheet for tooltips which looks like this:

QToolTip {
    border: 2px solid darkkhaki;
    padding: 5px;
    border-radius: 3px;
    opacity: 200;
    color: black;
}

QToolTip {
    border: 2px solid darkkhaki;
    padding: 5px;
    border-radius: 3px;
    opacity: 200;
    color: black;
}

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It's very annoying and I can't seem to get it to work reliably. I am not using a custom QToolTip object.

Note: when I build my app and run it in CentOS the tooltips always work. When I build it and run in it Windows XP tooltips only occasionally work.

Thanks in advance for any help you can offer. If this is a known bug, is there a workaround? Otherwise I am just going to have to shut off all the tooltips in my application.