Margaret, I have a similar issue: I likely have a single stage gas heater brut my nest learning thermostat will “jump” to stage 2 heating when the thermostat is cranked up, and it blows cool air and turns on and off repeatedly. My solution is to dial the thermostat back to current temp and wait till it turns gray. Then go up only 2-3 degrees from current temp, making sure it doesn’t say “heat stage 2”. Then it turns on and we get great warm air. I then keep adjusting 2-3 degrees up from current temp. Not elegant but it works.
I spoke to Nest support 3 weeks ago. After about an hour on the phone and lot of testing, Nest support person mentioned the white base section is malfunctioning. I found it hard to believe since i dont think there is much electronics in it. Anyways I relied on technical support and ordered a new Nest Thermostat, I didnt want to just change the base for $50. I installed new $2############rd gen Nest Thermostat and after 2 weeks I ran into the same issue where Nest is unable to get furnace going. Says its heating set to 72 while room temp was still 66.. says Stage 2 :-(... need to uninstall and return the new one, i guess nothing has changed. I will contact support again and keep everyone posted.