How is the station sited? Maybe you are getting eddy currents from your roof. I have seen badly behaving wind vanes where they are being affected by wind currents around and over the roof.
Greg H.
While entirely possible, I think this is more an AcuRite Pro Weather center (5-in-1) issue. As I click around of other stations on WU, I can tell by the wind direction data if it's a AcuRite Pro or not. Here are a few I saw today from various places across the country:
http://www.wunderground.com/personal-weather-station/dashboard?ID=KVASPOTS6
http://www.wunderground.com/personal-weather-station/dashboard?ID=KNESEWAR7
http://www.wunderground.com/personal-weather-station/dashboard?ID=KNESEWAR6
http://www.wunderground.com/personal-weather-station/dashboard?ID=KUTCASTL5
All of the above sites report the erratic wind direction. As I clicked my way across the country looking at the wind data, as soon as I see the history for the day and the plotting of the data I could tell whether or not the station was an AcuRite station or not before I even clicked on the "About PWS" to confirm
While siting could be an issue in each of these, my method for finding them was rather random (clicking on the map and checking stations). Most had fairly linear plots of the data indicating a general direction in which the wind comes from. But the AcuRite data is rather random and all over the place.
While my data is reported more often than most (Meteobridge reports for me) even those that broadcast to WU through AcuRite looked the same.