This sounds like a bandwidth issue between your systems and YouTube. I'd recommend trying to minimize the variables to determine what's going wrong. If you have a laptop, try moving to a different network to see if the problem continues. If you have a router inbetween your computer and the cable/dsl modem, try connecting directly to the modem, etc.
I'm having the same buffering problems with ver 11.6.602.180.
Just tried again during the same time of day.
Backed-out back to 11.6.602.171 and the problem no longer seems to exist.
I running Vista Home Premium SP2 32-bit on an HP Pavilion dv6500 notebook w/ 2 GB RAM.
AMD Athlon 64 X2 Dual core processor TK-55 1.80 GHz
Firefox browser 19.0.2