Audi Repair: 94 100S engine runs cold, catalytic converters, engine temp

Question
Hi,
My '94 100S runs cold, usually only when at highway speeds.  If I pull of this highway, stop the car, and restart, the engine appears to warm up.  Once I return to the highway, however, the engine temp plummets.  This is accompanied by a check engine warning.  Is this related to the ETC sensor or the thermostat?  This car has also been eating through catalytic converters ... is this a related problem.
Thanks for your help,
Colin

Answer
Yes this sounds like the thermostat has failed, which means the water is going through the radiator, which gets cooled very efficiently when going along at highway speeds.  The catalytic converter problem is possibly related since they rely on heat to be efficient, but I cannot be sure about this.  Change the thermostat first...  Let me know,  Jan