3 years ago our heater would shut off in lower temperatures. We replaced the heater on advice from a heater and airconditioner repair company , a few weeks later it happened again. Every winter since then it will shut down in the evening and early am and continue until the temp get higher. We changed the pvc pipes to 3 inch thinking that would help but it didn't. Very frustrating, it seems it must be something outside the system (gas heater)

Have someone check the gas supply pressures and regulators. You don't have a programmable thermostat that you forgot about do you?

The gas would be colder. That could be natural or cumbustion gas. If there is nothing unwieldy about your vents like 40' or 10 elbows, then I'd look at gas pressure as EP suggested.

Looks like the hvac company got richer.

Is this LP/Propane gas you have or natural?

Does the furnace have a status light on the front/blower panel? If so, it will provide valuable clues to the problem.