I finally set aside a day to do some serious A/C troubleshooting. I decided to first trace the wiring diagram to determine if any wires were loose. I found the nut that secures the ground wire to the compressor was missing. Although the ground wire loop connector was still on the bolt, it wasn't a good connection. I secured the ground wire and the A/C now works great!
I still have low voltage across the ignition switch, so I may install a relay as others have done to improve the blower fan speed.
Kim - 1987 Spider Quadrifoglio (~170K original owner miles)