(Portland, OR)  --  A man is undergoing a mental evaluation after engaging in a five-hour standoff with Portland police.  Authorities say the man was suicidal and was armed with knives when officers found him standing on a vehicle yesterday morning on Southeast 81st Avenue.  Officers responded to the scene after a report of a man trying to break into a home nearby.  Officials say the 53-year-old man surrendered shortly after noon, having suffered minor, self-inflicted cuts.