Disco 3 auto gearbox shudder

Hi all, I've recently bought a D3 TDV6 HSE auto, it's done 155k miles on a 54 plate but in excellent condition. I now have one niggle however and that's the shudder under acceleration, I have changed the gearbox filter and drained 4.5 litres of oil, when I refilled I got just under 6 litres back in so it must have been low! Seemed fine for the last 3 weeks but shudder is now back, worse if anything so I renewed the oil again yesterday, 4 litres out, 4 back in but no different this time. I know that I'm only changing half the oil each time but am a little worried there's no improvement this time. The oil I've used is Millers ATF DM which is their recommended oil for it. Any ideas please? Anything else i might need to look out for? Thanks in advance.

Is the gear change cable lubricated, they can get dry and then not move the selector fully