It sounds like your 'Flame Sensor' is not working. It sits in the flame
of the burner and sends a small electrical current that tells the furnace
computer that there is indeed a flame there. If it's defective or covered
with crud, it won't send the proper signal and the computer will think the
burner didn't ignite and shuts the gas off for safety reasons.

On my Lennox furnace, the part is a no brainer to replace.....two screws
hold it in, it costs about $15 for the part. But if you call a
repairman, you're looking at over $100, I'll guarantee! My furnace
was doing the exact same thing as yours and I got by with just cleaning the
oxidation/crud off of it. Since it's an intermittent problem, I'll bet
that's it!