Stuart,
2004...wow I guess it has been that long of time since this has been going on. At that time I was having the same problem. I did get a bad thermostat at first and that is what threw me for a loop. So after a couple different ones I got one that was opening/closing correctley. I tested them with boiling water in the kitchen.
I am leaning towards the gauge being off and will get my old meat thermometer to test it...but still think that the leaky radiator might not be keeping the right pressure.
