Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
When your Canon camera behaves in an obstinate way, it may be a sign it needs to have its obscurer settings returned to the factory defaults, to cure it.
But your camera is 18 years old, you say. If you have not treated it perfectly well at all times, that much age is enough to make one suspect, this is actually a control circuitry problem.
So, aside from trying a Settings Reset, and also the following, one should consider having it repair-serviced...especially if these measures fail...:
Try removing the batteries, and leaving the camera on a (dry, non-humid, non-dusty) shelf for overnight. Then put the batteries back in and try the camera again.
Questions about how well your camera has been cared for in its 18 years:
Has the camera been dropped or otherwise subjected to mechanical shock(s) in its lifetime?
Has the camera been operated in or subjected to, temperatures below 32 degrees F (freezing point) or above about 100 degrees F ?
Has it ever been subjected to a humidity higher than 85 % or gotten wet ?
Has the camera been operated in a gritty or sandy environment, such as a beach, a seaside, a construction site, or the like?
Let me know what you think, and whether you need any further information, please. I will advise you further as appropriate.