Do you see a pattern of where the spike is? Is it usually in the same spot?
If you're using your phone, there maybe spikes in between buildings or dead spots.
If you really want a more accurate way use bike computer with speed and cadence sentors.
I often saw brief spikes like that when I ran strava on my phone because the GPS unit was less than spectacular. Especially in the city. They've entirely disappeared since switching to a nicer GPS.
That being said, the spikes are usually much smaller than they appear on the speed/distance plot (generally they're less than 100ms in length) and so they don't disrupt the averages or anything except the maximum reading.