I have found that some YouTube videos always skip (i.e., stop and start),
others never skip, and still others sometimes skip and sometimes don't. Can
anyone explain this? (BTW, my computer is low on virtual memory. Could
that be a contributing factor?)

It could be a number of factors. Sometimes low virtual memory is a factor. Sometimes videos have problems when they are uploaded and that's why sometimes they skip. I also heard if there is a lot of traffic on youtube videos will take awhile to buffer.

I've had the most trouble with "Official Videos", meaning videos that are put on by a studio or some sort of company.

They always load slower and always seem to stop randomly to load.

Videos from "regular people" were always fine to me.

The HQ videos also makes things slower for me ever since it automatically has the HQ on.HQ used to never give me that much problems.

Without knowing a few things the like speed of internet connection and computer you have, it's hard to say. It sounds like you have an older pc with a hard drive that's filled up or your pc swap file/page file settings are too low.

I've had the most trouble with "Official Videos", meaning videos that are put on by a studio or some sort of company.

They always load slower and always seem to stop randomly to load.

Videos from "regular people" were always fine to me.

I have had trouble with those official videos too. I hate those VEDO type music videos.

I have had trouble with those official videos too. I hate those VEDO type music videos.
*VEVO

And yeah, I hate their advertisements and how fast they make YouTube slow. It'd a good thing as far as they have HQ video channels for many artists, but yeah.