No air flow from vents

[BONAS]

Hi everyone,

I got 2005 Murano MK1 Z50 3.5 4x4, I guess someone apart from me already had or have similar issue.

So here's the problem :

When I start the car in the morning (engine is cold) and turn the heater ON everything is fine (fairly), but after some time (around 10-15min), air flow from the vents drops down, specially if I'm driving on high speed on motorway, it's barely warm inside. 

What I've checked :

1) Fan is working perfectly fine,

2) AC pipes under the hood is frosted all the time, 

3) Firewall is boiling hot (I can barely touch it),

4) radiator is full,

5) Vents under the front window is clean,

6) I can switch between different vents (it's working properly), 

7) There is no change of air temperature when I switch between hot and cold air.

My thoughts :

1) Air pipes are blocked somewhere

2) Faulty AC controller

3) clogged cooling system (it has nothing to do with airflow) 🤷🏻‍♂️

4) One or some of the flaps are jammed ( I checked flaps on the side of AC and they are moving when I change between vents)

Anyone have any ideas or suggestions about this odd behaviour? 

Thanx guys!