87 300E a/c is acting poorly despite best efforts and limited acumen: (Make that a duh!)

A/C functions reasonably well while rolling down the road, but doesn't want to cool well at all while sitting at an idle

What I have done:
flushed multiple times
All fluids fresh
replaced drier/receiver and switches
tested pressure ok (exc on high side but not so on the low side)
no leaks
fan ok
new expansion valve didn't impact situation...

Rather than guess any further or simply throw parts at the impasse, thought someone here might provide a "Eureka moment" for me.
And yes, it's damn hot in Florida in the Summertime.....and humidity in between hurricanes is even worse....:-)

Thanks in advance for any help.

-j