Unknown object displaying unusual flight characteristics.At approximately 23:35 PST, I observed a small object at a relatively long distance, estimated at 3-5 miles, over the eastern section of Portland, OR. This area is roughly due east of the major airport in the area.The object accelerated at an unusual rate and performed maneuvers inconsistent with any aircraft design I am aware of. At times, it hovered in place, then proceeded to accelerate at a high rate in an east to west direction, then doing the same in reverse. In addition, it also appeared to descend rapidly, then ascend again over the treeline into view in the same location.There was no change in the lighting that you would expect with a turning aircraft, as the object appeared to maintain the same aspect at all times. At approximately 23:37 PST, it sank below the treeline and left my field of view. Unknown if this was due to a descent in altitude or acceleration away from my position, as both would produce a similar effect.