Using red wire for smart thermostat voltage
I haven't encountered the intermittent comment before, typically the R terminal in the furnace is +24vac, usually using red wire to the R on the stat. If using a tester on vdc setting the display will be erratic.
There is no code for color thermostats (or most other devices less than 50V).
But a trade standard many techs use Red for the R +24 hot , White for W heat switchleg, Green for G fan switchleg, Yellow for the Y cooling switchleg.
Historically there has been no use of the C 0vac common at the stat, so no real standard has developed, just using any left over wire or stealing the green fan control wire if only 4 wires are available and reterminating both ends of the green on C.
But if somebody has a cable with a different color set, like a 4 wire phone cable, any color swap can be used so it is absolutely necessary to document the old terminations before disconnecting the stat. Wire tags are good, pictures are great. If your documents fail you can usually recreate or confirm colors by looking at the furnace end of the cable, those terminals will be marked.