I told you in 44 to get rid of the MPH relay and power the original Guzzi relay socket direct from the battery through a new fuse.
You have a weak joint somewhere, I suggest you replace the wire all the way from the battery to the relay 30 and back from the relay 87 to the solenoid.
It's a well known fact that the original Breva wiring is suspect your troubles seem to have migrated from the original to the MPH kit.
If the factory bothered to draw the starter properly they might wake up to the problem.
To keep this new wire somewhat the same as the drawing you might like to make it yellow No 14
Make sure the spade connector at the solenoid is good quality and freshly crimped with some grease on the wire first.
Perhaps run this wire by the battery terminal so in an emergency you can touch it to the battery and hot wire the starter, if you do this slip a little tube over the bare spot.