I have a 15 year old Bryant gas forced air furnace. The furnace has shut down intermitantly this season.
It has an electronic ignition that locks out and the furnace shuts down.
When I flip the power switch off and on the spark will start and the furnace will operate again for days.
Any suggestions as to where I should look for the problem?

There are manypossible reasons, I would start with a temperature rise and air flow issues. If it is 90% efficient I would make sure all the drains clear too.