Iveco 75-15 temp gauge in red?

When I bought it I realsied the temp gaugue didnt work. We found the wire had been cut, so it was reconnected and everything seemed fine. Today it has been stood idling for about 30 mins and moved back and forth and I noticed the temp had gone into the red. The water level is fine and a friend removed the water cap on the reservoir about 5 mins after I had turned it off and it slightly hissed for a few seconds. I had expected it to bubble over if it had been that hot and Im wondering now if the wire had been cut because the gauge is giving a fasle reading or if it is actually overheating. Any ideas please???