Flame sensors and thermocouples are prone to failure, and will sometimes fail quickly if there are traces of organic chemicals contaminating the combustion air, otherwise they're usually good for a decade or so. If you can clean up the business-end of the thermocouple with a fine plastic scouring pad you might get a few months or even a year out of it. (or not- it depends.) More aggressive abrasives like emery cloth or 600 grit sandpaper may work too, but can create surface roughness that only makes the problem recur sooner.
Any bad electrical connection in the loop could also cause issues too.