The fact that your air con. effectiveness is effected by vehicle speed would suggest a problem with condenser efficiency and or condenser cooling fan operation. Low refrigerant volume could cause the same complaint but if your garage has not picked up on this then you can ignore this possibility. If your condenser is clogged or fouled with dust or other road related crap it will not allow the refrigerant contained within it to exchange heat with the atmospheric air flowing over it and a reduction in system efficiency would be felt. A good blow out with an air line from the engine side could ease the problem but the true cure is a full clean with the condenser out. If your condenser cooling fan is innoperatve insufficent air will be available to cool the condenser when at a stand still and a system high-pressure switch will cut the compressor and, subsequently, the air conditioning system as a whole. This is unlikely in your case because the ram effect of your car moving at speed removes the need for a pusher fan and the air con. should then work fine. Finally: A refrigeration system requires an ambient air temperature lower than the high-pressure temperature of the refrigerant. The closer the two temperatures get to one another, the lower the efficiency of the refrigeration system. If the external temperature of the condenser reaches around 50 degrees (such as when exposed to direct sunlight) the system simply cannot work. I have seen aesthetic modification cause this. Oh one last thing... E39 pollen filters (cabin filters) are particularly prone to clogging if neglected. The reduced air flow in to the pasenger cabin can, unfairly, give the impression the air conditioning system is at fault. The filters are located under the bonnet (hood) under two seperate plastic covers at either side of the car above the bulk-head (fire wall) secured by thin, hinged wire clips. Have a look and replace if necessary. Good luck and stay cool.