Clicking some links for first time result in error, subsequent links work ok

The first 3 links (at least) in the top left corner of In http://www.traveljust4u.com.au (Argentina, Bolivia, Brazil) don't work the first time you click on them, despite the fact that the links and pages are valid. But if you click on those same links subsequent times they work fine.

Any suggestions for this myster?

Cheers!

Error Message: Server Error in '/' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.