I'm attaching a few screenshots to this thread to explain in a bit more detail:
screen1.jpg - I'm routed through my main provider like normal and running a speedtest, which is running pretty much right at 5 megs, which is what I have the queue for my connection set at for testing purposes. You can see the speedtest.net window open on the left, and the Mikrotik window on the right showing the same throughput.
screen2.jpg - As soon as the speedtest finishes (and while still routed through my main provider), I start playing a Youtube video at 1080p, which should essentially max out the 5 megs, at least until enough of the video is buffered. The video stops to buffer a lot, and you can see that the video is only downloading at around 3 megs (and kinda choppy) even though I literally just finished a speedtest that pushed 5 megs without any problems.
screen3.jpg - I simply change my route to point at our backup DSL connection and restart the video, which plays fine without any buffering issues. As you can see it is downloading at 5 megs (the Winbox window happened to show 5.5 megs at the moment I took the screenshot despite myself being queued at 5 megs) and the download speed is much more consistent.
I've been working with my upstream provider (who is another WISP), and they claim they haven't really been noticing the same issues, but that's not to say they just aren't getting reported to them like my customers have been. I can't seem to find anything that would indicate issues along the way for my main provider. Like I mentioned, I should have more than enough bandwidth (If I'm at my office on a little better connection, I can test at 10-15 megs without an issue) and the latency is fine. No QOS that should be affecting anything.
I just can't seem to figure out what would be causing it. If I re-ran the tests tonight, the video may play fine, or it may not. Doesn't seem to be terribly consistent, which make it hard to track down.