water temp gauge fault? HELP

Hi i have a golf gti mk1 cabrio on a H plate 1990
the fault is that the water temp gauge is not going past 1/4 way over and when the car is warmed up and i get slighty over 70 the red light starts to flash but engine oil temp never goes over 100, heaters work fine as well as the fan and the water level it to the mark any idea's? thanks Tom