i have a 1978 GL1000. The temp gauge raises all the way to the HOt position (indicating over heating). Thisjhappens after about 15-20 min. of riding. The fan seems to be cycling fine, I have installed a new thermostat, coolant(no air locks) and a new radiator cap. I had to add about one cup of coolant last summer. What might be the problem. I have heard it might have something to do with the oil. Any suggestions?