ChrisB.

I hope i can explain this clearly. It makes no sense to me.

Lately my 08-C350 ends up intermittently blowing cold air out of the vents even with the temp turned up and the engine warm.

Needless to say this had been more than a little annoying in the cold weather. For a while there will be heat and then suddenly cold air?

For some reason it also seem to be worse while stopped and the car and the trans is in Park.

ChrisB.

Actually goes from hot air to cold are when I come to a stop. When I am moving the heat is perfect. Come to a stop and it starts to blow cold air.

kjb55

Are there any signs of coolant in the oil or oil in the coolant?

Dan Morgan

I am not familiar with the workings on that year and model, but I believe that the duct work is controlled buy vaccume. It is possible that while you are stopped the vaccume drops below minimum.
Does you car have a coolant recirculation pump for the heater core? If so, you may have a bad connection possibly.

benzmodz

Sounds like your TX valve is misbehaving. This valve is under the wipers and it controls the amount of hot water and cold gas coming into the heater box.