Ok, thanks for the extra info - a great help.

The problem could be a transmission internal issue, but the first thing to check is the shifter mechanism itself because if it has a faulty cable or worn bushes then that will cause your symptoms.

Cheers Glenn

Ok thanks for clarifying that,
as I said the problem could be an internal one, in any case this requires an accurate diagnosis, maybe a pressure test to see what's happening inside the transmission.
At best it may be a failing filter or a valve body problem.

I would recommend diagnosis be done by a transmission specialist,
I would suggest using a member of the Automatic Transmission Rebuilders of Australasia (ATRA). ATRA is a division of the Institute of Automotive Mechanical Engineers (IAME) and brings together a professional highly skilled and dedicated group of Transmission Repairers in Australia and New Zealand. Go to: http://www.atraonline.com.au & click on member lists to find someone close to you.

Don't forget that if you have any more questions or you are unsure of what I mean in the information provided please ask for more clarification, I'm only too happy to work with you to achieve a good result. Cheers Glenn.

The JustAnswer system is set up in a way that allows the customer to pay a deposit, and if you are happy with the answer supplied then clicking the accept button will process the payment. We strive for customer satisfaction and I believe that by a process of elimination I have provided the only logical solution for your problem but again you have to be happy with the information supplied in the answer.
If you don't click accept then the deposit remains in your JustAnswer account for future use.

Cheers Glenn