Was the new fuse box from another 97? As J57 says, if 97 was a 1-yr only, the connector configuration could have changed very slightly to where the temp gauge is connected to some other source with the new fuse box in. So engine temp isn't going up, but it's measuring something else that is.
I know from experience that GM does this. 00-07 Suburbans are all the same, but they changed the pin config in the trailer brake connector in 03. So you needed harness A for 00-02 and harness B for 03-07. The connector was exactly the same, but two pins swapped positions in 03, nobody knows why. Use the wrong harness and it plugs in fine, but trailer brakes don't work.
