Sounds like you are trying to sync the HD version of the TV Episode. You should try to download the Standard Definition copy instead and sync that to your device as HD content is not compatible with the 2G iPod Touch. See here for more information:
To prevent HD versions from being downloaded in the future head to the iTunes Store, click the Purchased link, and then select TV Shows and remove the tick mark next to Download HD when available. You should be able to download the SD version now from under this same page.