It's possible that the light meter that sets the aperure/shutter speed/film speed is showing signs of wear or damage.

Or it could simply be something mechanical like the shutter speed is no longer reacting correctly (less likely to be the aperture, although it is possible) ie the light meter decides it should be 1/250th, but it's opening/closing at say 1/350th so letting in less light.

Probably as expensive, if not more, to repair than buy a new one