Thanks for the reply, I appreciate the help. I was doing between 35 & 40 MPH when the light first came on.
Coincidentally, I just had the water pump and thermostat replaced during a timing belt job (might-as-well syndrome). Based on your analysis, I'm going to assume it was a bad/failing thermostat. So, this morning, I cleared the code and will wait to see if it re-sets itself after a few driving cycles. If it does, then I'll look into replacing the radiator and/or head caskets.
Thanks again for your help.
Regards,
Bob