i've never heard of them 'starting over', maybe your model does... does it have trip distance and trip time? because typically they ignore the stop time. ie if your speed is 0 they don't add add that into your average speed.
assume instantaneous acceleration and deceleration for the following example otherwise the math is more involved:
eg, if you do 10kph for 10 minutes, then stop for 10 mintutes and then do 20kph for 10 minutes your computer should show an average speed of 15kph even though in reality your average speed is 10kph.
does that example help or confuse things entirely
check your trip time when stopped, it should be stopped as well.