> Those last maybe 6 months and then they start getting dimmer and then all the lights in the circuit get worse at once.

Have you checked the wiring? I may be stating the obvious, but in household electrical wiring every fixture or receptacle is connected to its circuit in parallel.

I can't think of a way for one bulb on a correctly-wired circuit to fail in a way that causes other bulbs to dim, unless the failed bulb is essentially shorting out the circuit but not causing the breaker to trip for some reason.

If you know why it's behaving that way, I'd love to know too. I've never seen anything like that.