AC problem 89 300e

[maximus]

I am having trouble with my ac guys , when the temp outside gets above 75 to 90 degree the ac doesnt work but let say it is like 70 -75 my ac works perfectly very cold I took it to a mechanic to have it checked out he says that everything is fine he only had to put 1/4 lbs of freon in there but today the ac did the same thing the compressor kicks in ,works fine but no cold air is coming out of the vent .I just went outside start the car turn the ac on it works fine but the temp now is 72 but earlier got as high as 95 and i had no ac.

any help would be greatly appreciated! thanks guys