Problem viewing YouTube Videos

All of a sudden I'm having difficulties viewing videos on YouTube in particular. Video takes too long to load, frames are jerky, counter keeps going back and forth at the end of the video. Moreover, my Mac is very slow during the process and almost crashes if I try to maximize or minimize window, or work with other applications. The rainbow disc is almost always there. This happens only on YouTube. Could anyone please help?

N.B. I tried uninstalling and re-installing Flash Player. I tried repairing permissions (as they said on a Mac forum). I tried emptying cache and resetting Safari. I tried using TimeMachine to restore to an earlier time. I tried using MacKeeper to empty some space on the memory. I tried opening Safari in 32 bit mode, but it made things worse (videos were cut down to a couple of seconds). Nothing seems to work.

My System:

MacBook Pro 13"

Processor 2.66 GHz

Memory 4 GB

Mac OS X 10.7.4

Safari 5.1.7