Strange problem next to a gas fired forced nouns furnace.? The furnace is a Carrier, 5 years old. The thermostat is an...

The furnace is a Carrier, 5 years old. The thermostat is an elder type Sears brand with mercury switches.
It have been working fine until this later cold snap. ILL here.
The furnace works as normal except for some occasion.
Once in awhile after the furnace have finished it's cycle
it only stays stale for about a minute. Then the burners fire up again for a moment and the fan runs for going on for another minute and then the furnace returns to it's average routine.
It's like it have a hiccup once every few cycles.
Question for the plumbers...does Drano really corrode pipes?
Answers:    Sounds to me like your thermostat wishes replaced... Try using a digital thermostat, they are more accurate than the older mercury style...
Where can I find 5 gallon hose bottles...
nouns circulation seems to be the culprit check and verbs the air filter which may be fitted for the recirculation, is chocked.