BMW Repair: BMW air conditioning, bmw 520i, fan motor

Question
I live in Turkey and my 1995 BMW 520i was imported from Germany.  When I use the air conditioner for 1-2 hours, the air flow is much reduced.  Even at level 4 the air flow is reduced to level 1 or less.  But the fan motor still SOUNDS as if it's at level 4 and what little air comes through is still cold.  Then it's better to turn the air conditioner off and open the windows.  After about half an hour, I can turn it on again and the air flow is restored.  This is good for about another hour or so, and then the same thing happens.  My mechanic is baffled by this.  He says he has driven the car for at least two hours and the air conditioner works perfectly.  Yet for two summers now, it has consistently failed during long hot drives.  What do you suggest?  

Answer
The a/c really reduces the humidity in the car,thats why it feels cool.Your mechanic may not have the fan as you did thats why he feeels no problem in the operation.Have all the vents checked for any blockage in the vents and make sure the a/c is fully charged.
There may be a problem with the fan motor as it doesn't want to blow faster.Having read your question again I might consider checking that.The motor sounding as if it is turning like level 4 may be becauuse the bushings in the motor are allowing it to wobble in the housing