My current internet speeds according to the xfinity speed test is 90 down and 15 up. Whenever I am playing video games and watching YouTube videos at the same time my ping will always spike, even when the video isn't at HD resolution.
What are your system specs? It almost sounds like your system can't keep up with the demand. How about if you scale back the quality of the youtube videos? What is your router? Is it decent? Maybe it's the bottleneck.