First check the guage for correct temps using another guage of some type for a comparison. FIx the damn leak, it will bite you sooner or later...
As for the temp changes it has factors of ignion timing, water pump, thermostat, cooling fan, the flow of the colant, engine bore, spark plugs, loose fan belts. Most small block chevys were ment to run 190 degrees and as high as 215 degrees temp. on newer cars