Why does the electric furnace sometimes blow cold air

Turned out to be the High Limit switch. The switch had burnt out, which caused the burner not to fire. Turns out when the limit switch senses the heat box is too hot, it shuts down the gas and forces the blower to continue so it will clear the excess heat. When the switch failed it always told the system that the furnace was over heated, so the burner would not come on and the blower would run constantly to try and clear the excess heat.

To trouble shoot this issue I used an ohm meter to test each sensor, to make sure it was open/closed as it should be. Another way to test would have been to use jumpers to bypass the sensor all together, until the faulty sensor was located.

Have you checked the temperature inside the furnace? It is possible the pilot light went out.

Disclaimer: I don't hold any special knowledge about furnaces. They are dangerous if tampered with. Re-lighting pilot lights can blow your house up. I don't recommend anything.