Try opening the podcast in another video player - (in iTunes) right click on the episode and choose "Show in Windows Explorer" from there open with another player (Media Player Classic is a favorite of mine). If you experience the same stuttering, it's the podcast's fault. If the stuttering clears, QuickTime is too taxing for your processor (it needs time to literally "catch up").
Another possible culprit may be that you're playing the podcast from an external drive and the disk's read speed is slower than the video playback speed.