From your description, it sounds like the metering cell is responding as it should, except that the aperture in use is not being accurately transmitted to the meter except at the wide-open position.

When you change aperture on the lens, the meter coupling ridge of the lens moves the meter coupling lever on the body. I believe that is attached to a variable resistor that is either failing or whose internal wiper is dirty or worn.

It needs the attention of a competent technician to restore it to correct operation. For most of us, this is not a do-it-yourself type of repair. Without the proper tools and knowledge, any attempt to “fix” the problem is more likely to make it worse.