i just had this the other day.. bloke said it was over pressure and the compressure was *****d. i'd topped it up with a recharge kit using an inline gauge, so i knew how much pressure was in it.
anyway this A/C expert was pricing up a new compressure and dryer. i noticed these rubber flaps on the fan cowling. which didn't seem to be doing much. i tried to open one just to see what would happen and it snapped off in my hand.
when i checked the rest they snapped off aswell. and the air con temp dropped quicker than a whore's knickers.
i assume that because the rubber had harden and the flaps couldn't flap the air from the leccy fan had no way of getting past the rad and condensor, so it couldn't cool the gas and there by giving a high pressure that was too high.
so the point of all this wittering is.... if you have high pressure and no coldness check to see if you have hard flaps, it has the same effect as a blocked condensor