I've noticed the same problem somewhere since the rc1 release. Mostly on the news tickers and sometimes in movies when the camera moves right or left.
Since i made a complete reinstall of my system when i installed rc1, i figured it was a driver or decoder problem.
Tried diverent ati video driver versions and setting vsync to always on didn't solve it.
Ati avivo, pdvd7 and 8 decoders didn't made anny diverence.
Setting te priority to high as sugested in this (verry old) thread didn't either.
Finally solved it by running mediaportal on one core instead of both cores, should've tried that a lot earlier since i used to have major problems before with mp on both cores when tsreader (-use tsreader if i remeber correct) was stil in the test phase.
only problem with this is that you have to manually set it to one core on each restart of mp, wich is i'm glad to say only needed if i'm updating to a new svn. as mp runs rock solid
Running smooth as ice now
ATI HD 2600 Cat 8.5 720P@50Hz (pal)
AMD64X2 3800
PDVD8
Windows Vista