One more question about curses/scrolling

[Lite Lin]

Hello,
  Yesterday I posted a couple of questions regarding curses/scrolling,
one thing that I mentioned was that the screen "trembles" when I try to
scroll (on a SUN shelltool/cmdtool/vttool, using the SYS V curses package).  
I'm now closer to figure out what the problem is but still no solution.  
The bottom part of the screen "trembles" only when there are blank lines 
on the screen; otherwise scrolling is done just fine.  Any clue?
  Thanks,
	Lite