It sounds like you might be trying to sync HD videos to your iPod classic.
HD videos purchased from the iTunes Store are compatible with iPhone 4 or later, iPad, iPod touch (4th generation or later), Apple TV, and computers. You may need to download your purchased video from iTunes in the Cloud in a lower resolution for your device.
You'll want to check out the following article to resolve the issue, specifically the "Download previously purchased content in a lower resolution" section:
Have a great day,