I suspect that you may not be running at 230 degrees with a 7 pound cap without some discharge from the overflow down pipe.
Assuming that you are using an electric gauge, I would suspect that it may be defective. The easy check would be to replace it. Easy to do and inexpensive (as far as I know?) You could use an infrared temperature checker, but I don't know where to tell you where to aim it to verify the correct temperature.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission