Anybody heard of this? Ever since it's dipped into the 50's in the mornings, I've tried to use my heater. I let the truck warm up in the driveway before heading out, but even after several miles of driving, the heater will only blow warm air for a minute before the air turns cool. Yesterday, I was pulling a moderate load for an hour and the heater still wouldn't stay hot. Do I need to get a winterfront for this thing or is there a problem I need to see a mechanic about?