tooltip paint problem in win8.1

I have a fully "owner drawn" tooltip control I use to show popup help in my programs. I create it with WS_POPUP | TTS_NOPREFIX styles and nothing else fancy
  
 then I take over completely its painting overriding WM_PAINT and WM_PRINTCLIENT to show my own visuals whenever the tooltip pops up
  
 all this is working fine in older windows (with or without visual styles), but with windows 8.1 it fails to paint properly, intermittently. It looks like this
  
 http://tinypic.com/view.php?pic=15hedth&s=8#.U4yXHUr6i01 http://tinypic.com/view.php?pic=15hedth&s=8#.U4yXHUr6i01 
 
 parts of the tooltip don't draw and remain transparent. Around 70% of the time it paints ok but sometimes it doesn't (it always fails in a different way)
  
 I know that newer windows versions don't use WM_PAINT as older windows but I wonder why it would fail intermittently like that?