Problems with my AU II auto trans not changing into overdrive and not changing back to 1st when stoping. But it will come back to first if you manually pull it back with the shifter

I'm new to all this and my AU gearbox is also a bit temperamental.

I've already dropped the oil and changed the filter. I'm now contemplating dropping it again and cleaning out the valve body and all the solenoids.... Is your oil level correct and is it clean?
For info you can get a new filter from a high street parts supplier who are CHEAP and SUPER about $16:00 from memory, they also stock the Castrol TQ95 oil. I'm guessing the closest for you is Albany or Busselton. I've also bought an Autel Maxidiag US703 scan tool (bought on Alibaba direct from china-cheaper than Ebay). This has thrown out a few fault codes that have been of a help to me. I've also checked out a few post on that online free sales site. TREE GUM, or something similar. This lead me to a bloke in East Cannington who breaks AU (in his backyard) from him I've pick up a secondhand BTR gearbox for $100. Incidently if you drop the oil pan and find little pieces of brass. Then your transmission oil cooler in the radiator may also be on the way out (found a few in mine). I thought they were part of the gearbox itself but when you see the inside of the oil cooler you can see that they're from there. The closest reference I can find to the oil cooler inner is a course stainless steel scouring pad my missus has on the kitchen shelf. I believe it's used for one of those household tasks. I think it's called washing up.

Hope this rambling is of some help!