Cooling is better at high speeds if the fan is not working properly or is
just not designed to give adequate airflow at idle. Make sure the fan
always kicks in when the A/C is turned on.
Open the hood and see if the compressor engages at idle. It should,
otherwise everybody stuck in a traffic jam would complain about lack
of cooling (not just insufficient cooling).
In 90-102 degree weather, the compressor should run pretty much
continuously unless it is low on its refrigerant charge. It might be worth
it to you to take it to an A/C specialist and have them hook up a gauge
set to see what the pressures are like on the low and high sides. In
really hot weather like yours, I wouldn't expect the low side pressure
to ever drop much below 30 psi.
You could also just bite the bullet and pay for the system to be evacuated
and recharged with the precise amount of refrigerant the system calls
for (it should be on a label on the underside of the hood), and then given
the high and low side pressures for a properly charged system at whatever
the outside temp happens to be for future reference.
One last thing you could do all by yourself is to get a thermometer and
measure the temp of the air coming out of the vents when the A/C is set
to recirculate. It should be at least 30 degrees below the ambient air
temp
measured near the passenger footwell where the air is drawn in, which
means only 72 degrees until the car cools down more.
You should also check that when the A/C is set to recirculate, that no
outside air is being drawn in. Have somebody spray a little Lysol near
the cowl (not directly in it) while the car is sealed up and the A/C is
on max. You shouldn't be able to smell it if the outside air damper is
working properly. In really hot weather, recirculate (Max A/C) is the
only way the A/C system can keep up with the high temps and humidity.
Bob