Spikes could be from poor satellite reception through the dense clouds and GPS making corrections(assumptions). Or they could be from a major straight wind front that keeps squirting you forward. I wouldn't think that you would be able to double your speed in quick bursts like that without either pitching or breaking something (mast/torn sail). Would be cool to see the track overlayed with a dopler radar image. Maybe someone at Garmin is working on that feature.

-Rob V.