I have a 79 f150. It was dumping coolant out of the overflow when i shut it off so I put aftermarket gauges in there and new coolant. It was fine until today on my way home from work sitting in traffic my temp jumped up to 230. I removed the thermostat whhich seemed good and drove it and it's still overheating. The previous owner put water in it instead of anti-freeze so the radiotor has that rusty color to it inside. What could be causing this and how can i fix it. P.s. when I rev it up when its hot the gauge goes back down to about 190 which I think is normal.