Update: it looks like, at least for me, much of the issue is/was on my end.

Over the past week, I've been having some issues with my internet connection (Fios, which I still love). Here's the deal:

I kept calling Fios support, and sadly because their 1st tier is not trained to know about my setup, none of them could really help me.

See, in what they call "Multi-Dwelling Units" (MDU's), if the construction company doesn't let Verizon run fiber to the units, instead they'll install an ONT in the telephone closet on each floor, and run VDSL from the closet to the apartment. So I have a DSL modem/router in addition to the standard Verizon router.

Unfortunately, when I call the tier 1 people at Verizon, the second I say I have a DSL modem, they either think I'm a moron or they don't have any clue what to do next.

It wasn't until the modem died completely that I was able to get someone sent out, and fortunately Verizon's installers are very familiar with my weird setup.

In addition to the modem, it seems there was a messed up connection in the telephone closet, but now everything is fixed and it seems like Youtube buffering has improved greatly.