If the intermittent occurrence seems to be ambient--inside--temperature
related, maybe cold spray in a can or nitrogen from a distance to cool
gas valve or circuit board might bring failure. That is assuming it only
happens at night when you're not there. If it happens on warm
days, try a heat gun on each component. Note that over-cooling,
like coating with ice, can make most any circuit board at least temporarily
fail, so don't go crazy. Me, I haven't actually tried this in HVAC, but
it was commonly used in electronics troubleshooting 25 years ago.
I recently bought a heat gun to try on one customer's intermittent problem,
but then we had a couple days of relatively warm weather and the gas
valve wouldn't work at all.