It appears that either your connection to YouTube is being throttled in some way or it's a congested server.
As YouTube uses a different codec in Chrome than Firefox, I would suggest testing Firefox (or Internet Explorer) with a few HD videos. Chrome uses the VP9 codec, where as Internet Explorer and Firefox both use the H264 codec, so YouTube will likely fetch the videos off a different server.
Another possibility is HTTPS based connections being throttled. One simple test to check is download a large file from a HTTPS connection see what throughput you get. One example is the standalone Chrome browser package which is served from a HTTPS connection:
Based on your above speed test, you should get a similar speed to what I got:
If this downloads very slowly at the time YouTube is having trouble playing in HD, then it's very likely HTTPS based traffic is being throttled on your ISP. Otherwise, if you're just having trouble playing YouTube in HD, it's possible YouTube is trying to play from a congested server or along a congested path to it.
see if your speeds here on TMN are slower during the same hours. mine typically are slower by 10-20% between dinner time and 11 PM or so. it's a little worse than it was a year ago when i got on the 50 mbps down and 5 mbps up plan at Time Warner. i've noticed that it's still pretty fast in the morning up until 11 AM or so. from 11 AM to 11 PM it's spotty. one minute it's full speed, the next minute it's way down (sometimes to 60-70% of max. there must be a lot of people streaming video