Have you had the AC system serviced?

When the freon load starts dropping the system stops providing cool air to the pax side- so a classic symptom of low freon is the side-to-side variation.

You also state the system has trouble cooling in hot environments- another sign you should have the overall AC checked out.

Very hard to DIY- proper way is to evac the system, determine freon mass, recharge to a specific weight. $100 at any competent AC shop. If you HAVE lost some freon, have them put tracer in- next time youll be able to see leaks. Add $15