Bright green light, erratic movement

Location 38^48'20"N, 121^13'38"W near muni water tank. Area is largely residential, foothills, with some undeveloped parks/greenbelts. Weather calm, skies clear. No local haze or smoke layers. Bright green light was observed jerking in the sky, east of my vantage: up.stop.down.stop.left.stop.right.stop.pause...again, not pattern evident or assessed (I was too mesmerized by the event to anylize). Event happened less than 0.5-mile away. My static position was at the same relative elevation as the ufo. The motions and directions of the ufo seemed random, "stop" phase varied in length from 5 to 20 seconds (estimated). "Go-Stop" happened very quickly. The individual accelerations could not be correlated to any aircraft that I have previously observed or flown in: accelerations seemed to exceed the limits of conventional aircraft and bio-limits of humans (4g). The UFO dropped below my vantage at several times. I could not witness any ufo contact with the ground (undeveloped) at these times. I do not know when the ufo event started prior to my discovery. I watched for perhaps 10-minutes, then the ufo dropped below my vista and I did not see it re-appear; I abandoned my vigil. This ufo was really bright, less than 1000-ft altitude (earth approx. 300-ft alt at this location), near populated areas and provided a prolonged exhibition. I am hoping to discover other witnesses to this event.
Professional Engineer