The problem you are running into is due to the use of usercontrols in DotNetNuke.  In fact, it is not really a problem, but a feature.  In your example you have a module with a textbox named txtValidText.  Lets say that ASP.NET didn't prefix your control with dnn_ctrl377_.  What would happen to your script if someone placed two instances of your module on the page at the same time?  You would end up with a control array.  To work around this problem/feature you need to have your script be aware of the UniqueID of its container.  This information (usercontrol.uniqueid) can be sent down to the client through the getVar/setVar functionality detailed in the ClientAPI document (page 9-10) or in some custom way.  However, you need to still think through what happens if two modules exist on the same page.

Hope you find this useful.