Intermittent Connectivity with Macbook Pro

I've had this issue for a long while now. My lightning cable for my iPad works 100% fine when the device is connected to a charger and charging from a wall socket, but only SOMETIMES will it work when connected to the Macbook.

I've tried various usb ports on the mac, I've tried the lightning cable both ways in the iPad.... sometimes it will connect immediately and work perfectly fine, but other times it simply won't. It just repeatedly disconnects and reconnects, so quickly that the Mac does not even register its connected to it. You can see however the lightning bolt in the battery disappearing and reappearing every half a second, and the constant sound effect of disconnect/reconnect coming from the iPad itself.

Its bizarre to me that the cable works fine on the wall charger, and works SOMETIMES on the Mac.... why not all the time? What the heck is going on? I've cleaned both ends of the lightning cable, making sure there's no grit in any ports or the like. I'm genuinely baffled.