What a relief! The past few weeks I was having the exact same issues with youtube as the original poster, with the original poster's exact same internet plan. (As well as Firefox.) You Tube videos would start, play for 5-10 seconds then freeze up. Drove me CRAZY. Asked a tech person at a Future Shop on Thursday about it, and he said it was a common complaint with MTS users with my set-up, and that resetting the modem usually fixes it. So that's what I did.
The only thing was, I couldn't remember what it meant to reset, exactly. Just unplugging the modem for a few minutes, or actually pressing the reset button on the modem first, then unplugging it. So I did both. After I did, You Tube started working again flawlessly, as if by magic. With one caveat. Now I get ads with You Tube. I run Firefox, and I'm unable to get the Ad Block Plus feature to block them, no matter what Ad Block settings I select. It's not such a big deal, as I'll take a brief ad over the videos pausing every ten seconds. I did also notice a recent Adobe Flash update (Macbook Pro) and maybe that had something to do with helping to fix You Tube, as well. I'm generally clueless about computer things, though, so I apologize in advance if Adobe has nothing to do with You Tube.
**Oh, another thing I noticed is that the problem slowly seemed to creep up once I moved up to the Lighting plan. (Which was about a month ago.) Never had a problem with You Tube with the slower plan. It also didn't matter what other devices I had running at the same time. I tested You Tube with all my other devices turned off, with only my Macbook Pro running, and I still had issues. However, after the reset, so far so good. I also tend to empty my Firefox cache every other day, as well. I'll report back if I start having problems again.