It's difficult to say but it could be due to extra processing required on the video stream. If things like deinterlacing are required and the video chip doesn't support it, that can cause heavy CPU load and cause choppy video. It can also be caused by CODECs that don't support hardware acceleration for such operations. Poor decoding options in players can also cause choppy video.
I looked into Intel video issues some time ago. Other than performance, there was a reason why Intel 4400 video was a better choice than earlier versions. Don't know if 4000 and 4200 video suffers from the Sandy bridge bug but that could have been an issue. The 4210U CPU is over a year newer than the 4210Y so it's not likely to suffer from earlier Intel video issues. Some of the chip configurations on Zotac boards are quite dated so they need to be examined closely.