[janmustang]

When furnace ignites only 2 of 3 burners ignite therefore the flame sensor detects no flame and shuts down, then goes through a restart. Sometimes it works sometimes it goes into self heal mode and a few times has gone to lockout mode of 1 hour and has to be reset. Sometimes when it ignites 1+2 ignite and 3 is delayed but does beat the 8 second window to start unit.

[bitters]

Sound like the burners need to be cleaned.

[Grady White]

If your burners look like small airplanes, the 'wings' are what carry the fire from one burner to the next. It is quite common for them to become plugged with rust or other debris.
Remove the burners, wire brush each one, clean the slots in the wings with a very thin putty knife of a blade type feeler gauge, blow every thing out with air (canned or otherwise), put things back together, & give the furnace a try.