Getting any CEL codes? If not, probably an issue with the cluster.
I'd recommend doing datalogging and reading the coolant temp as seen (ok, well, inferred) by the ECU. If the temp is reading reasonably, then the sensor and wiring are ok and its a gauge issue.
Remember, there is no real water temp gauge, but a CHT sensor instead; the connector is on the back side of the block, with the sensor under the intake on one of the heads.