Youtube have been doing some maintenance lately, changing servers and stuff. They had some details on their site about it a few days ago. Also some videos do take a while to load depending on what server the video is being fed from. Youtube have also started working with higher quality files which could also be slowing things down.
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I've also tried reinstalling flash player and java and also turning off my firewall and it still has the same problems.
I'm not sure there is too much we can do about it, if I find any new useful information about it I'll let you know but hopefully youtube will get this problem resolved soon.