Pressure/Temperature light stays on...
 

I have a 1976 2.7 911 with some 70K miles. Have had it for about 7 years and everything has been fine. Recently the warning light for temperature/pressure started turning on when the car heats up (normal heat up of the engine), and it does not go away even when the engine revs up... when I first saw it I got really concerned, but then looking at the temperature and oil pressure gauges in good ranges, as well as oil level, I kept on driving; this has been occuring for a few months now... and it does not ease my nerves! anybody has had such a problem? should I just simply change the bulb, or may there be any other reason for this situation? I also changed the oil but no improvement. Thanks for your help