Water Heater problem

Do you have the potted circuit board? What I have found over the years that was baffling to me also is that just because you have that correct voltage does not mean the correct amps are getting through. I would try a jumper wire directly from your battery to the fusable link the next time it won't fire and see if it consistently fires it should be the brown wire. You can compare the amp reading with the jumpered wire to your original wire this will tell you if you have corrosion somewhere or a failing switch. After playing with my WH for 10 years my board finally failed the test Old Biscuit posted and I replaced it with the Dinosaur board and it now lights every time. I also installed a new wire and switch.