Lennox Furnace with Electronic Ignition - Pilot lights up...keeps on clicking but furnace won't fire up.?

I have a lennox G8Q Model furnace -1991. Its functioning for the most part, however, when there is a call for heat the igniter engages and sparks up the pilot light. The pilot light heats up the sensor (glowing red hot) but the furnace does not fire up all the while the electronic ignition keeps on sparking (as if the pilot light is not on) then after a period of time sometimes 20 sec sometimes 15-20 minutes it fires up. I think its the flame sensor but I'd appreciate any help / suggestions.