Now that fall has arrived we've had to start turning on our furnace to keep warm at night (old house as well). I'll set the temp to what I want it but it's all over the place during the day or evening and then at night when we really need the heat, we've got to krank it to over 27 just to get a decent amount of heat. Some nights it hardly gives anything, other nights it's boiling. There doesn't seem to be any regulating. Could it be our thermostat? We had our furnace checked last winter, it's about 5 years old and they said it was fine. The thermostat is an old "Carrier" with only one control switch and that controls the temp. Any ideas?

Sounds like the thermostat to me.