MacBook Pro - Macbook Pro not playing YouTube HD?

Hey I've got a late 2007 model Macbook Pro 15.5" 2.33gHz model. I've upgraded my sdram to 4gb and gaming is no issue, similarly with watching HD video offline, key word being offline.
For some reason when I'm on a website with embedded video like YouTube or anything along those lines, when a video is offered in HD I simply cannot watch it. To save some people's advice here's what happens and what I've done:
-video acts as if it's constantly hitting the buffer and isn't done yet
[yes I've clicked HD then dragged the cursor back to the beginning and let it load before playing but it still happens]
-audio plays and buffers normally
-severe lag with the video but stays in time with the audio
[i.e. if the video lags the screen is black and when it comes back is in sync with audio, it's not like it doesn't match]

What can I do to solve this, if anything?