I did a few tests this weekend. First i ran a jump wire from the sensor to a good ground which made no difference - the gauge still went to 230' as soon as the key was on. Then I disconnected the wire to the sensor and as expected, it went back to 0. Then I quickly took the Red/Black wire to the sensor and grounded it and as expected, it pegged the gauge again at 230'. Then I changed the sensor for the one that was in the car when I bought it and there was no change. The ground strap on my ZF is clean and I also added a thick ground strap from the starter to the chassis and I don't have issues with other electronics in the car related to a bad ground.
So, the next step will be removing/testing the temp gauge - Any pointers on which steps to go through in doing this would be very appreciated. I have pics of the wiring on the back of the gauge from when I installed it and it appears correct based on the SOBill wire diagrams. I am a little suspect of the in-line resistor - any help would be great - Thanks