Do you have an electric gauge?
I know it's a pain, but try a mechanical, I had an electric temp gauge that was reading wrong (50 degrees high :o) and it scared the daylights out of me (new engine build). I tried a cheapie mechanical (Exxis IIRC basically store brand from B&C / Carquest) and it read almost dead on (checking with an infrared temp gun, the casting of the intake where the temp sender was was within 8 degrees of what the gauge said).
