The starter solenoid would like to have 50 Amps at the spade connector, for some reason yours is getting less than 20 making it unreliable.
Its either too small a wire from the relay to the solenoid or too much resistance from the battery Plus to the (3) relay pin 3.
I suspect the wiring on yours may be a little different in that the Start Relay pin 3 is fed through the ignition switch as they do in the later GRiSO and most other CARC bikes.
The fix is to make sure the relay pin 3 is fed directly from the battery through a 20 Amp fuse[You must be registered and logged in to see this link.]
The breakdown guy proved there is nothing wrong with the battery or starter by bypassing the faulty wiring, this is something everyone should be familiar with, the only thing you have to be real careful about is making sure the bike is in neutral, pull the clutch for good measure.
BTW the starter solenoid is nothing like they show on the schematic, there are two coils, one draws 10 Amps the other draws 40
Heres something you can do to identify the wiring on your bike
Pull the Start relay out of the socket then probe the socket pin 3 with a Voltmeter or test light to Chassis.
Is there power there with the key turned Off or do you have to turn it On?
Here is the other thread I started [You must be registered and logged in to see this link.]