1996 Range Rover Starter Motor

I have an intermittent problem with my starter motor, sometimes it works ok and other times it sounds as if its not engaging the ring gear correctly. The car has been standing for a while without much use and the starter motor has been replaced. If I start it a few times in a row it gets better starting but if left for a while it then seems to be a problem to start?

Any ideas if this might be an electrical fault or mechanical?

Many thanks

Matt