As squarebear says, the pause is while more data is buffered. Potential causes are:
Time of day - if you are on at the same time as lots of other users, the YouTube servers will be stressed at that point.
If you are on at the same time as everyone else in your area (say, when you first get in from work or school) the local loop will be stressed in the same way.
If you always rush to watch clips that have been recommended on a popular site, you'll also hit a problem.
But the problem could be entirely local to you - have you checked your speed lately? Have you got some real cr@p protection software (eg Norton) which is causing a bottleneck on your machine?