A/C problem 300CE

[Early Bird]

I have a 90 300CE and the A/C has not been working right for over a year now. Its R12. Mostly blowing slightly cool air. Lately it has been blowing cooler air, BUT, when its Hot out like mid 80's on up it blows hot air. So it actually seems to work real good when its very cool out side like early morning and in the evening and not so good blowing hot air when its hot out.. Any ideas. Refrigerant has been checked and found to be at proper level using the guages. Sunday it was about 91 or so outside, my Wife and I were going to head to the coast, but the A/C was blowing what felt like 90* air, so we jump into Her Japanese car which is like a freezer when that A/C is used.. I don't expect mine to be that cold but those Japanese A/C's you can't beat.