I am an A/C - Refrigeration Tech / Company Owner. I would have to say that was a low pressure switch. As the compressor turns faster, the LPC (Low Pressure Control) will kick off the compressor by opening the circuit (continuity) on the common wire to the system.Then, when the compressor is off (by clutch) the pressure then rises again allowing it to turn back on. In your case the low pressure switch is not allowing it to come back on and cycle like its supposed to. Though - If your going fast and the condensor is cooling quickly (due to fast air flow on the condensor coil) the pressure should balance out at higher speeds and not have to cycle as much.
You could actually cut the wires going to it (its only 2) and connect them together (as the switch is)make sure its insulated like a wire nut, and that compressor will never turn off ! Its a device connected to the suction line - or "low side" , its the cold one. On your town car I think its right next to the accumulator (the big round cylinder)
Also, another possibilty is a HPC (High Pressure Control) and it does the same thing the LPC does. There is a possibilty there is a big bag or something like that covering your condensor coil. The fan will work at low speeds but at high speeds the fan is turned off because your getting alot of wind from the front and the fan is no longer needed.
I saw XLRVIII said it was a low pressure switch, how did you know? Is there a post i missed ? haha
Any A/C Technical Info Needed Send A PM. Id be more than happy to help.