Where's the frame ground located?
Several things to check:
1- Engine ground, located at the other end of your battery negative wire, look from the foot brake side of your bike, left of the clutch lever and towards the center, the starter uses the outer case as negative connection, several have had a corroded engine ground
2- frame ground, on the MK-3 there are 6, the most important one is on the right side of the bike, just behind the seat / tank mounting bracket, my money is on a bad frame ground, if MK-3, remove the tank / seat mounting bracket as there is a second frame ground to ECU directly below the mounting bracket, taking the 2 bolts out, you can slide the mounting forward to gain access .
3- check both battery terminals,that is remove the connections, tip, put a small piece of cardboard or hardboard under the terminal nut before you take the bolt out
makes it much easier especially if you added accessories to the positive terminal to get the bolt started again. Clean with scotch brite pad or similar.
4- the start solenoid next to the main fuse under that triangular cover , left side almost directly above the foot rest, one screw holding the cover in, several have had the relay fail, doubt that is your problem.
5- have your battery tested, it may be near the end of its life, measure the voltage when running at idle and after about 5 minutes of sitting after a ride. You could also hook a meter up to the battery before starting and see what voltage it drops to while not starting.