AirPods Max disconnect and left cup crackles with movement

My AirPods Max headphones keep disconnecting. When I put them on, they’ll either not connect at all or the noise-cancelling will momentarily kick in, the left cup pops/crackles before they completely stop working for a few seconds, the ANC kicks back on again and then it fully connects to my iPhone or iPad. However, if there’s the slightest movement of the left cup, it goes through it all again. I’ve seen that some people have said there’s sometimes buildup of some sort of black gunk between the headband connectors and the cups, but have cleaned this and the cups. Nothing has sorted it.