Can't Run Scans

I periodically run into times when I can run any scans at all?  This happens probably at least once a month and sometimes lasts for several days.  I've had Norton help remote in to my computer and they thought they had the problem fixed (took them almost 3 hours), but less than a month later it started doing the same thing.  Does anyone know what could be causing this and what a resolution is?

Thanks!