Unit is 21 years old. In auto when a call for heat occurs, furnace will light then after several minutes shut down on what I presume is the high limit because the blower isn't getting a signal to run. However, if I flip the t-stat fan switch from auto to on before furnace shuts down then the blower runs and I have hot air. T-stat is a 6 year old digital Duracraft with fresh batteries. I've never had a problem with it. A week ago the unit ran fine in AC mode. A cursory inspection by my untrained eyes couldn't find any loose wiring connections at the t-stat or at the furnace. Furnace is burning nice blue flames with no CO in exhaust or airstreams. A tech is coming tomorrow to fix/repair. Any opinions on what is wrong?