Intermittent Starter Issue

Mods please delete this

My thots: if it actually was the starter at fault, it would not be intermittent. You've more likely got a poor connection somewhere. That connection could be a relay, a connection at a connector somewhere. Check all your battery connections. Actually take them apart to make sure they're clean. Specially the ground connections.

You need to take voltage measurements at various points along the path from battery to starter motor to see where any significant voltage drops occur. You said you checked the starter relay connections, but did you verify there isn't an abnormal voltage drop across the terminals of the relay? If your mechanical connections look OK, the next step is to measure voltages, that may help reveal your problem.

Clean the switch with plastic safe contact cleaner from the auto parts store.
There are two sets of contacts in there. One to turn the lights off the other to pull in the starter solenoid.
Would hurt to clean the kill switch while you're there.

err looks like you already did that. Did you clean up the contacts with a pencil eraser while you were in there?
might want to check the voltage drop across the solenoid. Could be dirty contacts there.

When the dash lights dim and the starts doesn't turn what is the voltage at the battery?

Old school "fix" was a healthy whack with a persuasion device on a starter which sometimes moved it enough to get the brushes to contact.

Yes it sounds like you may have a dirty kill switch. Try cleaning the electrical connection there.

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Don't rule out the starter yet, could be the brushes need replacement. One or more of the starter's commutator segments could be a source of the problem. Next time you encounter the failure to crank the engine, try shifting the trans into 4th or 5th, then "rock" the bike with the clutch engaged (ie, lever not pulled in) so the starter will rotate a few degrees, then see if it will crank over and start the engine. If it does, you've narrowed the problem down to the starter.