ipad keeps disconnecting and reconnecting from mac

Bit of an annoying one this. Checked a bunch of threads online as to what the issue may be and can't find a fix.

Running a 2014 iMac and just picked up an iPad 3 (this issue was happening with previous iPad 2 also).

When the device is connected to the Mac via USB it keeps connecting and disconnecting at a rate of about once every half second.

Because of this I can no longer sync, back-up etc.

USB ports are working just fine as evidenced by iPhone 6s and other peripherals which connect fine.

There is no VM software on my Mac to conflict with.

Tried restarting countless times, different cables, switching off Push and many other 'fixes' I found online.

Of note - with my old iPad 2 that shared the same problem - I managed to get it connecting quite randomly to my gf's PC laptop.

But there MUST be a solution to this surely?

Hoping someone might be able to shed some light on this issue!

Thanks in advance,

Dave