Problem with streaming video on Win7 HP x64

Good evening. I wonder if anyone can help me.
Back in April, I reinstalled Win7HP from scratch on my PC, using MS's iso file. I previously had the 32bit version, and I now have the 64bit version installed.
Everything has been working fine (although I don't notice any significant improvements in performance), except for one thing: When browsing Youtube, video performance is much, much worse, particularly HD vids. It's virtually impossible to watch a vid in 1080p; they just freeze and hang constantly. 720p just about works, but only just.
I have exactly the same hardware setup as before, and all drivers are up-to-date, so I'm not sure what the problem is. I have an ASUS 512Mb graphics card, and the fastest fibre-optic b/b connection.
While googling around trying to see if others were experiencing this issue, I did read that Adobe Flash Player only works with 32bit software, and I am now using Firefox's 64bit browser. Could this be the problem? (But I thought that Youtube now uses HTML5, and not Flash, in any case??)
Many thanks for any suggestions/advice.

At long last I seem to have found out what the problem was, and I'm gonna share it because it's not something I would have considered before I read up about it.
Basically, when I reinstalled Win7, one of the tweaks I performed was that I set the Windows virtual memory (paging file) to a fixed size of 4096Mb - the same size as my RAM - rather than the size Windows recommends by default (1.5 times the RAM). I can't remember why I did this, but anyway, one of the Microsoft knowledgebase articles I was reading suggested increasing the pagefiile size, so I changed it to the recommended setting, and Bingo, problem solved, instantly! HD vids now buffer and replay fine.

For years, if not decades, I had always thought that Virtual Memory is something that Windows only uses when all the RAM is in use. Turns out it's much more complicated than that. Many applications cache stuff to the pagefile FIRST, and then transfer the required bits into RAM when needed.

Anyway, I thought that other readers might be interested to test if this is the case on their own Win7 systems too, because if so, then we may have stumbled upon a very important piece of advice for anyone else experincing video problems.
So, if anyone would like to play a HD (1080) video on Youtube, then reduce your paging file to say half its current size, (or to 4096Mb), then try and replay that vid and see if you notice any problems.