There are poor connections somewhere. My suggestion would be to shut off the mains and methodically go outlet to outlet, switch to switch tightening all junction points. I also always bend the ends of solid conductors to wrap around the screws. I never use the insert junction points. I’ve never trusted them.

Edit wrap the conductor clockwise.

Second edit, sorry, while the mains are off make sure the wires are tight at the breakers or fuses. Do not try to tighten the incoming mains. That will hurt, maybe forever hurt.