I've been trying to solve a issue with Youtube video's crashing my internet connection for a few months, and finally figured out what the problem was. I run AVG virus scanner - the paid version, not the freebee - which has a built in Video Accelerator. Apparently Youtube doesn't like this. Disabled it as follows and Youtube problem went away.
You can disable it in AVG Tools -> Web Browseing Protection-> Online Shield -> untick "Enable AVG Accelerator".
This is your destination in the 2013 version of AVG. I don't know for sure, but the same or similar problems may occur with other video accelerators, so if you're having this problem try disabling or removing the accelerator.