Trane Burners Fail To Stay Lit

[krawfo]

I have a 15 yrs old Trane VX80 gas fired furnace Model 4TXCC043BCHCAA
Recently the burners will only light occasionally.  The red LED on the control board goes from slow to rapid flashes when the thermostat calls for heat.  The igniter glows and the first of 4 burners lights but the other 3 fail before the igniter stops.  Sometimes all 4 burners will light after the 2nd or 3rd try and other times just the fan comes after after 3 failed tries. 
I have cleaned the flame sensor twice with a Scotchbrite pad but still can't get the unit to consistently light on the first try.  The filter is brand new.
I'm ready to call a service tech but am concerned that because the unit will perform normally for 8 or 10 hrs he may not be able to see the malfunction while he's here and then have it fail after he leaves.
Thanks for any help you can offer.

[Admin]

You should have a pro check it out.  A crack in the heat exchanger can cause the problem you are describing.

[krawfo]

Will do.  Thanks for the suggestion.

[Porcupinepuffer]

I've seen this caused by dirty/plugged crossover sections from the burners. When they're rusted shut, the flame cannot travel to the furthest end where the flame sensor is.