1995 Vauxhall Omega temperature gauge faults

My Omega runs great with the temp gauge around 90degrees then all of a sudden it starts to go up to around the hundred mark stays there for a few minutes then usually comes back down then a few miles further down the road and off it goes again, there is no sign of water leak and no water in the oil any ideas?? it had stayed there around the hundred for about 10 miles on one occasion , I stopped the car let it cool down for half an hour and it was back to 90degrees again then ten miles down the road up it went again stayed there for a mile or so then down it came it started doing this about two weeks ago