Gill, if you download & then open your track in GPSAR, then compute speed statistics, you will see if you have any abnormally high speeds (my best speed today was 133knots over 100m, which I`m pretty proud of..)
Then, you select "show graphs" and select the right 3 boxes, ignore alt.
Under Trajectories, slect the ACTIVE LOG, then look at the bottom 3 graphs, if you see "spikes" in the graph, select your speed that makes no sense (if there is one), then hold shift down and select delete under trajectories. You then re-calculate speed statistics, and see if any spikes still exist, and go through the process again until you have a clean track. If you go to www.gpsspeedsurfing.com you might find a better description than I have given, but this is the process I followed to get rid of my 133knot top speeeeeed.