Air conditioning not working

My car AC works very well when not needed, i.E. When its colder outside. It literally blows cold, very cold air. In the event when the outside temperature or inside temperature is very high, AC fails to work. It blows warm air.

Checked the Freon pressure and found filled.

Sometimes it starts colder and then in couple of minutes (5 minutes) it starts blowing warm air.

there is no noise or foul smell.
By the end of last summer, I went to the mechanic and they vacuumed everything and filled it again (both at H and L). It was working well for couple of days but again the very same story.

Please advise. I am sure this would have been asked earlier as well so please point me to the right blog to look at.