I have a 2005 Silverado 1500. The heater blows fine but only heats intermittently when I drive the truck. When I start it, the heater may or may not heat up. If it does, it may stay warm for 2 minutes or an hour. It will typically alternate several times in a 60 minute drive. Can anyone help me?

Thanks in advance.

 

sounds like an air pocket circulating around....or you are having a problem with the temp blend door in the dash