mboxCurrent is not an object that exists in at.js, which is one of the reasons we have documented at.js as incompatible with the legacy integration. The newer A4T integration is compatible and customers should upgrade.
If your company is no longer using the legacy integration in any way, you should deactivate this response plugin in Target Classic. If you are not sure how to do so, you can add this to your client care ticket.
Is your test rearranging the elements in your top navigation? I do see some flicker if i stare at that portion of the page.
A few side notes:
You are using the legacy T&T-SC integration which is not supported with at.js. You will see an error in your browser console "mboxCurrent is not defined" which is related to that. Do you have a plan to decommission that integration when you migrate to at.js?
To correct one of Shekar's notes, redirect tests created in the VEC are now supported with at.js v0.6.4 which is the version you are using. We have updated the Help documentation to reflect this.
I wasn't able to see flickering when loading your page. Though I'm not sure exactly what to look for. Are you using the target-global-mbox to serve the test content? Or are you using a custom mbox location?