G’day and welcome. From what your saying the default setting in a btr auto is third. It will lock into third if it’s not getting any speed reference from the speedometer. Are you sure it’s not knocking back from fourth into third? Otherwise if you are certain it is going from third and into second ( it would be revving the engine at around 4,000+rpm ) you may have some serious transmission fault.
Trust me to own the only unreliable AU on the planet haha. In saying that its only ever not gotten me home once when the PS rack pressure switch let go an hour from home. I'm just going to put some new plugs in it and fill it with some thick oil and drive it until the utes done. then ill make the desicion to either put in the lowish Kilometer VCT ticky motor I've been offered or just get rid of it. I've done 100 thousand in it so it doesn't owe me much and the body's rough and it has a few other issues with it. Would hate to take it off the road but I'm fairly bored of it and dont like the idea of spending enough money on it to be able to sell it and then not get much cash for it
Welcome to the forum, there's lots of knowledgeable guys on here feel free to start yourself a build thread (with plenty of photos) if you want to, if you are browsing from a phone I can highly suggest getting yourself the tapatalk app makes uploading photos very easy, if using a pc hosting sites such as imageshack are great way to upload photos then link them across, as the forum only let's you upload very few photos directly, as far as you auto goes it will most likely be a bad/dodgy solenoid causing the problem, someone else who knows transmissions better than me may be able to shed some more light on the subject