The fact that it relights right away - and then runs fine for some time (not sure if it does it a second time after 5 or more minutes - usually gets up to temperature fairly quickly and then shuts down normally) leads me to believe it is not likely cycling on limit, or flame sensor??
I have not cleaned thesensor. Filter is kept clean and the problem has happened even when the filter had just been washed (washable electrostatic)
Would this be "normal" behaviour if the furnace had run 12 minutes on low fire and then went to high fire?? Does the gas shut off (momentarily) when the gas valve switches from low to high??