OK so I checked the resistances and following the AutoMeter tech'* guide over the phone determined that it is the gauges black box that is faulty.
The bad news is that this gauge is obsolete.
I bought it in Dec/06, ran it for two months and now it'* obsolete.
Autometer tells me that due to low sales volume of this type of gauge, they don't sell them anymore. New temp gauges do not have an intermediate black box that sits under the dash to connect the gauge leads to and the sensor leads to. Ten connections in total.
The new gauge they sell is not the same as the one I have.
Autometer will send me a new black box but have told me that it will not be calibrated to the gauge so may not be accurate.
Now I am in waiting mode till the mail arrives.
Good to know it'* not the sensor so I don't have to tear into the engine again.