AC does not work with ambient temperature above 90

Intermittent air conditioning operation is usually not related to the refrigerant system itself - meaning, likely not low in charge of refrigerant. It could be an electrical concern with the compressor coil and clutch, a cycle switch or a relay. You could also have cooling fan issues intermittently perhaps. It's going to take hands on testing while the problem is actively occurring. Clearly when the system is working, it's going to be impossible to see what isn't right. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops