[mythtv] Horizontal 'seam' on HD (SVN 8343)

[Kirby Bakken]

When watching HD (either live or prerecorded), I occasinally see a 
horizontal 'seam' .  Its at about the '45%' (from the bottom up) point, 
and kind of zig-zags up to the 50% point, and then goes away.  This 
happens either with or with out XvMc (I'm running an onboard MB Nvidia 
6200 vidao adapter, feeding my Panasonic TH-42PWD5 (852x480).  I'm NOT 
running GL sync...  my cpu (again with or without XvMc) is so overtaxed 
that I get prebuffering pauses.....  Plus, the horizontal 'seam' doesn't 
go away. 

That's my 2nd question...  why does Gl sync cause SO much CPU to be used?

Any ideas?  Is more information needed? 

Thanks!