I also have an Altima (2005) and it is having the same problem with the cold air at Idle. My vehicle has 30K miles and the dealers solution is air pocket removal, antifreeze breakdown and a 30,000 service schedule. I am wondering about the waterpump vanes being worn. At a higher revolution, more fluid would be passed through the heater core. Just a guess. Has anyone been able to come up with a solution?