W9 intermittent click when starting, how do you fix it?

Ok, I've been dealing with this for years. I have a few W9's, they all do it. You turn the key to "on" the fuel solenoid clicks, turn key to start, just a "click". Back key off to "on", start again, turns over, fires right up. Sometimes you may need to hit "start" 2-3 times before she turns over. My '84/3406B, '96/N14, and a '96/3406C have all done it for decades. Batteries, starters, cables have all been changed over the years, not to try and correct this issue, just needing replacement. This is intermittent, happening anywhere from once a week to 6 months between, no rhyme or reason why. These 3 have never failed to fire up with at the most a couple of clicks with the key. Here's my problem. I have a '97 with a N14, does the same thing with the exception that sometimes it wont "catch" no matter how many times you hit (20-30-40 times) the key. End up pull starting it and it wont do it again for 6 months. Any one ever successfully diagnosed and corrected this?

 

Hadn't suspected the key switch. They will all start hundreds of times without issue. Then like I said, no rhyme or reason, click, click, click, vroom. Won't do it again for months. Switch is cheap and easy enough.

 

Its aggravating. There is no way to troubleshoot. It never fails, it will click, I'll get my meter to check voltages, hit the key and it starts. Its my '97 that Is really frustrating. Today was the second time it wouldn't start, just click. This truck parks at our customers yard. There's no time for me to gather up some tools and head over, the driver is already needing to hook and book. I'll have one of the other guys pull him off so he can go. It never does it when I have it here at the shop, smh.