I recommend these articles to try and resolve the issue you are experiencing. First before anything, I would recommend restarting both the computer, and the router/modem if you have not already.
Next I would suggest resetting Safari:
Safari 6 (OS X Mountain Lion): Reset Safari
Then I would check for 3rd party plugins and remove them.
Safari: Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues
The last resource I can find for this type of issue would be checking your DNS servers, but only do this as a last resort.
Safari 5.0.1 or later: Slow or partial webpage loading, or webpage cannot be found
All the very best,