Heater / Climate Control Issue

[SonofaBish]

This is my first post on here, but i've scoured the internet and can't find any other places where this came up, so i'm hoping someone has an answer

Its getting pretty cold here in PA... When i go out in the morning and start the car, the heater blows fine regardless of what setting its on (Auto or Vent)... heat blows nicely to both sides of the car.... Then, once the cab is warmed up, for some reason the passenger side of the cab starts to blow cold, no matter how high I turn it up - even to the max of 84*.... I checked the rear vents, and the same thing is happening - on the driver's side rear, it continues to blow hot air, but the passenger side rear begins blowing cold....

It doesn't matter what temperature i put it on, nor does it matter whether its Vent or Auto... Also, I HAVE gone into the settings in iDrive and changed the red/blue to all red, so that shouldn't be the problem...

As a final note, all the vents blow cold, even the ones to the floor

Cliff notes: heater is great until the cab is warmed up, then the passenger side of the car (front and rear) start blowing cold, while the driver's side (front and rear) blow hot as they should....

Any ideas/help?

Edit: forgot to mention - its a 2007 X5 4.8i

i would normally guess that its some sort of blend-door issue, but since it blows nice and hot at first, i'm at a loss..... researched the entire inter-web via Google and can't find anyone that has had the same issue