@Tok'ra
Having no GPU issues outside of Safari, i don't think it's hardware related.
Those of us speculating on whether the tab closing/switching freeze may be a hardware issue are only having issues in Safari. Xgrep, with experience working in the hardware industry, believes it is despite the limited scope of software triggering the freeze, so I remain open minded to the idea.
Did anybody experience the freeze without involving YouTube tabs?
Since there's a variety of different freezes being discussed here you may find a variety of answers that don't tell you much. For the tab closing/switching freeze I could describe it a little better as a GUI freeze that occurs when a page containing a HTML5 video embed is redrawn or 'undrawn' (not knowing the right term for that). I've described it as switching or closing tabs but I can also redraw or undraw a Safari page by switching to another application while that page was in focus. This has occasionally caused the same GUI freeze.
In terms of HTML5 videos, I've only noticed it happening with YouTube videos. The only other HTML5 video that gives me odd behaviour is Vimeo embeds which display a glitch-like pattern when they're in an unplayed state, instead of the still image preview they should. Playing them returns them to the normal, expected behaviour. I haven't had one cause a freeze yet but stopped using Safari some time ago.
If I remember correctly, you mentioned the type of freeze you were having a few months ago and it was just Safari freezing, not the system or the GUI.