Any Guidlines For Locating GPS Interference?

I work for a wireless provider & we have a device that's totally Dependant upon GPS. We've been intermittently losing signal for days at a time. Any recommendations on gear that can be purchased to at least identify the source of the interference. We've used a spectrum analyzer but it doesn't appear that there are any interfering frequencies. What other options are there for resolving a loss of GPS sync issue. The unit sees no satellites what so ever. When I test my Garmin in the same area GPS intermittently drops. (Strange thing is this device had operated previously great for over a year in the same spot that it's currently having issues in.) Any help would be much appreciated!