Jackaroo Gearbox !!!!!!HELP!!!!!!

Thanks for all your help - here is the latest

Thanks for all the advice and suggestions - We took it to a GM dealer today and all they could offer was a $95 plug in to the computer to test the electronics - no mechanical info. They also admitted they know little about Jacks, only see about one a month for servicing - Interesting - This is a big GM dealer in Parramatta.
They then offered to give us the name and address of transmission specialist they use.

Went around there and what a difference - plugged some computer in immediately and established it was not electronic.
Offered to service it immediately which we agreed to.
Result
In their opinion it had never had oil serviced or replaced, very small amount in it and that had the wrong smell. They then too off the sump and inspected things.
Filter was totally blocked
(Is this a part of the GM servicing program - the car has been serviced by GM since new?)
There is a magnet in the sump housing and it was covered in grey stuff METAL
Asked me if we were planning to sell it.
Offered (if need be) a rebuild for $2400 including parts and labour.
They replaced gasket, new filter, filled it with oil and set us on our way.
We are now waiting to see what happens over the next couple of days.

One interesting thing we noted - There is a magnet to collect shavings etc in the sump - Presumably it is expected that that will happen - My question is - Is that normal.
They also told us the car will tell us before it gives up - Either with warning light or further funny things like we have just experienced.

It's not the dealers fault that the transmission dopesn't get serviced, as the books say it's a "sealed for life" transmission. It has no dipstick and the procedure to check the oil levei is a bit of a pain.

Some argue that once you replace oil in a "sealed for life" unit, it will stuff it because, presumably, the clutches and gears have some sort of intelligence and will self destruct in this circumstance. Has to be cr@p, in my opinion.

My jack has 120,000k on it now and I have a tranny filter and gaskets ready to go in with new fluid. Haven't decided whether to take it somewhere or do it myself, since I don't trust that a service place will know how to fill it correctly and I'll have to check their work anyway. Don't want to be halfway to Bamaga with camper trailer on and a blown tranny due to not enough oil.

As pointed out above, the inhibitor switch can cause jerky changes. Why it does this is a mystery, but dirt ingress is a possible cause. Mine did this a couple of years ago so I took it to Barloworld who removed the switch, cleaned it and replaced it. It was much better but not right, as I still had an occasional jerky shift. So I pulled the switch off myself, split it open and checked inside. The main contacts are embedded in resin and the sliders are housed in recesses, so there is no possible way that they could misalign or bridge and short the wrong contact. But the process for installation onto the tranny is intricate, requiring a little pin to be inserted to align it, a bit like aligning a clutch. The position mine went back on was different by a few degrees, and there has been no problem from it since, so I can only assume that the dealer didn't know how to re-install it, or ignored the manual.

As others have mentioned, semmingly unrelated problems can cause flaring. I recently replaced the non-standard muffler with a standard one, which increased the backpressure again to approximate factory values. The result has been better changing from the Auto and better fuel economy than with the straight through muffler. Conclusion - the correct backpressure improves the computers ability to tune the engine which interacts with the powertrain control module like it should and improves gear changing.