A: There are several possibilities.
How are you calibrating the sensor? Are you calibrating it in open air or against a standard cylinder of gas? If somehow the cylinder or calibration method is flawed, theses are the results you might get
the other possibility is a damaged sensor (you mentioned earlier it might have frozen?). That might have damaged it somehow, although I would not say it is likely.
It could however, be a generically just bad sensor.
the other possibility is somehow the electronics are damaged or defective.
One other thing to look at is the interconnect wiring. There are four wires from the sensor (two for the sensor itself, and two for the thermistor). If the thermistor wiring is intermittent, it can do exactly what you say happens. This should be checked carefully.
Another possibility is RFI. If there is a strong RFI surge, it can couple into the wiring and cause a spike lasting a second or two.
