iTunes is splitting one podcast into multiple podcasts. How do I fix it?
Maybe the podcast moved to a different URL. A program like MP3Tag can show and edit extended properties (looks something like PodcastURL). Check the splinter episode and check its URL. Check an older episode. If it has changed, select all older episodes and edit the field. Click OK. Go to iTunes and Get Info for all of those episodes. When it reads the new URL, it should stick them all back together.

Redownloading the older episodes, if available, will accomplish the same thing. (They'll be downloaded from the new URL. But the tag edit is quicker and easier.