next time it does not light go to furnace befor cycling the thermostat and look for flashing lights in the lower door area, these are trouble codes, just count the flashes.most times the flash codes will be printed on the ignition control box.
the codes will tell what type of problem the furnace is having.
in the lower door of the furnace there is a small plastic sight/glass hole, looking inside this you can see the led on the ignition control, it will flash in counts of one, two, three, ect...
if it flashes two times, it would mean the air pressure switch is having trouble.
does this make sense?
if not let me know.