First place to start are the battery post. Remove, clean and tighten them. If the issues are the same, make sure that the wire to the starter is tight. There are a lot of parts in the starter circut, however from your description I doubt it is any of the safety switches (clutch, kickstand, engine emerg.stop,) so that leaves the starter relay. Check the wires going to and from it. It all of the wires are tight, You will need to start checking voltage beginning at the starter while it is engaged and then start working back toward the battery to find out where the voltage drop takes place.