That certainly doesn't sound normal. Actually easiest way to ensure things are playing synced is to quit iTunes!
If there's an encoding problem with the video it may affect sync or streamed video equally.
Try checking iTunes for a free TV Show episode and try that.
You need something encoded 'Apple's way', no matter how badly, to act as a benchmark - if that still plays up it's probably not the way your videos are encoded.
How about trailers for films etc on the AppleTV1 - do they play ok?
YouTube?
Have these videos ever worked properly in the past - if so could be a hardware problem brewing.
I've never synced so many photos either - if photos are acting as screensaver change it to somethng else.
If problems persist and all your media is safe and backed up you may want to try removing this itunes library from the AppleTV Settings menu and re-adding it again but doing so will wipe content from AppleTV itself and take time to resync.
Doesn't sound very usable as it is for video.
Do you have any 'hacks/mods' installed?
AC