I had a shwin (althought it was wired) and found the spacing between magnet and sensor had to be really tight, to the point of rubbing. I shimmed the sensor towards the wheel using scrap wood and electrical tape (oh yeah it looked pro).

That 80+ mph when the bike wasn't moving... was the magnet lined up with the sensor at the time? If so that's probably the sensors detecting/not detecting the magnet intermittently and reporting impossible speeds.