When did you start noticing the change in view frequency? I noticed around a similar range for average views, with a few videos here and there having a bit more, but not really a pattern (obviously, I can't see views over time.)
I checked out a few videos and thought that the content was very good -- informative, yet funny. I would say a few things:
For the Christmas/Saturnalia video, the branding is a bit inconsistent...so, you have a title saying "A Christmas History" but your thumbnail doesn't have anything overtly Christmas-y...and the text says Saturna-what. Someone who listens will have that explained, but if someone were just trying to find out about Christmas history, then they wouldn't necessarily make the connection.
I think one thing you could do is, if you have Photoshop, remove the background and then put something Christmas-y. Alternatively, I would include something Saturnalia-y...or even better, I would include something of both (and retitle the video to include something of both [for example: A Christmas History: Ten [or whatever] Facts about Saturnalia])
In terms of videography, it seems that there could be a wider variety of angles and distances in the video. You already do this *sparingly* (e.g., you have a bit at 0:49 seconds where you have a close-up clip to say "everything" and you do it again when you're considering the legality of murder, and you have a zoom in segment at 1:36 [which was AMAZING, by the way]), but I think this could be used more than simply as a situational technique.
You do have some text appear at some times in the video, but I think you could also make more use of images to illustrate various points.
In terms of Youtube SEO, I think that if you could upload closed captions for your videos, that would be great. Youtube can auto-generate closed captions, but this doesn't help from an SEO perspective. On the other hand, if you submit your own, then youtube will crawl the script in the same way that it would look at the title, tags, and descriptions.
For the tags, I think that you're missing out on some long-tail keywords. When you have a keyword for something like "history", for example, that implies that you're aiming at audience members who are just searching for the word "history". But this is likely to be a very competitive tag. You don't just want to target people who are searching for "fun facts"...but people who are searching for "fun facts about Christmas". Even if the latter is less searched that the former, more specific long-tail keywords will likely be easier to rank on the first page for.
(In your analytics, can you see in Traffic Sources if you're currently getting much traffic from youtube search? If so, what are the search terms people are using to get to your videos?)
