I would vote for the flame sensor as well. At least it is worth a look. I have a Carrier gas furnace, but it looks very similar to what KrK posted, and I have to clean the sensor every other year or so. Same symptom. A quick cleaning with some fine steel wool fixes it for another two years.
Question: Do you have a small window in the lower panel through which you can see a red LED. When my sensor acts up, it will cause the LED to flash a code (the cryptic deciphering messages that correspond to the flashes are printed on a sheet on the inside of the lower front panel).
On mine, if it fails three consecutive times, it will go into lock-out for three hours. In the dead of winter, it can get mighty cold in three hours; that is usually when I discover that it needs to be cleaned.
One word of caution. Be sure to secure power to the furnace if you decide to try and pull out the sensor to clean it.--
Shine on you crazy diamond...