jquery is undefined?

jquery is undefined?

In IE I get the following error 'jquery is undefined'.  I can not find any consistency that may be causing the error.

In FireFox a blank alert box just comes up with no related javascript error and any  jquery methods just stop working.  I'm assuming it's the same error.

I restart the browser and the error is gone. It always comes back.  Any suggestions as to a possible solution, a way to track it down or what may be causing the error would be greatly appreciated.

Thanks,
~Donavon