What one could do is for the guys that have trouble to do an ongoing ping test to an external ip-address (e.g. local newspaper) for the duration of the event. If one experience problems one can see if it was an unstable line that caused it. If the ping test show no lost packages during the time of trouble we can rule out the local internet connection.
Next step would then be to do the same test towards the server during the next race etc etc.
Next step would then be to do the same test towards the server during the next race etc etc.