ipad intermittently connects and disconnects with MBP over USB

When I plug my 6th gen ipad to my 2015 Macbook Pro via USB it will only stay connected in intermittent, half-second bursts.

All OS and itunes up to date.

I've tried several leads, all Apple, no improvement.

Other devices (iphone 6s, external HD, memory sticks, mouse, keyboard, etc.) connect to my MBP, no problem.

The ipad charges normally from a power outlet and the charge port is pristine.

When I tried plugging it to my girlfriend's vintage 2011 MBP the connection worked perfectly.

So, both devices work fine on their own and with other kit, but are unable to connect properly to each other.

Help please!