YouTube videos are just... not good.

Question

I watch a lot of YouTube videos.  I used to have really slow Internet, and we upgraded on July 23.  My speedtests went from 1.86 down, 0.3 up to 17 down, 1.5 up.  Everything else I do on the Internet has very much benefited from this upgrade, but YouTube hasn't, really.  For about two days, it was great.  I could watch a video in 1080p full screen and it wouldn't stop at all, whereas before I'd always had issues.  For some reason, Flash wouldn't buffer more than like 30-45 seconds ahead of where I was watching.  (This started around January or so.)  I had always paused videos at the beginning to let them buffer all the way through, but it got to the point where, for whatever reason, I couldn't do that.  I had to watch every. freaking. video in 30-45 second chunks, even on the lowest quality offered for that video.
Anyhow, so upon upgrading our Internet, it flew for about two days and then this problem came back.  It's not as bad as before - I can watch a video straight through in 480p or lower.  But I want HD!  If my Internet connection is 10 times as fast, this shouldn't be happening.  I should be able to watch a 1080p video full screen straight through like I was for those two days.
Info:
I have the AT&T U-Verse 18 Mbps connection, and it speedtests at 17 Mbps down, 1.5 Mbps up.
I have a PC that I purchased last summer.  (HP Envy-14 Beats Edition, model 1260se)  It has 6GB ram, an i5 processor, and discrete graphics.  I use Firefox and Chrome.  Firefox is currently my primary browser, but I'm working on switching to Chrome because Chrome is soooo much faster.  (I have a lot of tabs that I keep open in each and I don't have time to switch them all.)
I also have a MacBook Pro that I purchased shortly after this summer's release of the new MacBook Pro (with the retina display.)  Mine is not the new one, though.  It has 8GB ram and an i5 processor.  I use Chrome exclusively on the Mac.
Both computers encounter this problem.  And no, I don't need to reboot, I turn off both computers at night.
I have checked Task Manager on my PC while this issue is happening in Firefox to see if something is weird with my computer, and everything matches the norms EXCEPT for the fact that the Flash plugin keeps consuming 10-15% of the CPU.  While that's not enough to significantly slow performance of the rest of my computer, I wonder if it has something to do with this problem.  Neither Firefox nor Flash are using more RAM than they should be.
Also, both before the Internet upgrade and now, every time the problem gets REALLY severe and Firefox stops responding, then if Firefox does start responding again I see a message that the Flash plugin has crashed.
I just upgraded my Flash player on my PC to 11.3.300.268 (and rebooted, though it didn't instruct me to) in order to see if that would help, but no such luck.  The upgrade changed nothing.
Also, I just noticed... when I right-clicked on the YouTube video I'm trying to watch on my PC (I'm typing this on my Mac) the final option in the menu still reads "About Adobe Flash Player 11.3.300.265."  What's with that?  (Oh, and then right after I typed this, Flash decided to crash on me.  FUN.)
Considering the amount of time I spend on YouTube, I would really like for this issue to be resolved.  And to reiterate, I have fast Internet and two fast computers.  Everything else works wonderfully, but YouTube doesn't.

Answer 1

The change to YouTube's buffering policy was deliberate and done by Google/Youtube after extensive testing that showed them this provided a better experience for more customers.
Chrome comes with it's own version of Flash Player, that's why the version you installed (268) does not appear when you right click in Chrome.  If you tried the same thing with Safari or Firefox you'd see 268.
I'd recommend also posting this on the YouTube forums to see if they have any advice.