Altimeter no where near accurate

The altimeter is all over the place. Not a few meters here and there either (i dont care about that). Todays run I 'climbed' over a 20km ascent in a 19km 2 hour run! Im in the suburbs, not mountains.   Ive cleaned the watch and done hard resets.  Any further ideas?  Or even how to redo the runs already to reflect accurate hills?