Most modern GPS units do have already smoothing algorythms to do away with the so called spikes, but due to our slowish speeds the actual GPS positional error can cause unprectable results. Have a look at the actual NMEA data stream if possible and you can see an additional field which gives the actual GPS error at the time of the sample or just eliminate the really fast sample points and do a bit of " smoothing " yourselves to get more accurate tracks.