I have a Smith GV100W propane furnace/boiler. It runs great, but have been having a sporadic problem. About once every week or two, the furnace does not properly ignite. The blower turns on and will keep running, but the ingnition sequence does not actually light the furnace. I usually don't realize it until the temperature in the house dips by 5-10 degrees. I'll go down and turn off the furnace and turn it back on so that it can run through the ignition sequence again and it will then light. Any ideas on what would cause the furnace to not light, but still run the blower? Thanks in advance.