Very helpfull thread for me guys, thanks.
I just came from the local dealer garage (Geneva, Switzerland) where I was told that they don't change the oil in this gearbox. I asked them to just check if everything is in order.
My 159 TI is doing the 70.000Km service and I'm not sure they would check the gearbox in a normal factory service...
I've been feeling some strange vibration on highway at around 100Km/h (the gearbox sometimes chooses 5th other times the 6th gear) and when this happens the car starts vibrating when in acceleration... it disappears when I rev up the engine a bit to 120km/h in 5th a couple of times
The gearbox has "oil for life" so it is never checked or changed during the service. You can get the oil changed by a specialist transmission repair company or other specialist, but it is not a DIY job unless you have the right tools, documentation and very good DIY credentials!
The most common issues (as stated before in this thread) are sticky solenoids. This can be relieved with an oil change, but only a full refurbishment of the solenoids will solve the problem and that is a £2000 job. This is most noticeable as "up shift flairs" in the lower gears (usually 2 to 3) and "clunky" down shifts. An oil change will make this a thousand times better.
The thing with this gearbox is it is very sensitive to torque changes out of the engine. If your engine is running rough and producing less power than normal, the gearbox will not be fully calibrated to this reduced power state and as such can struggle to get enough torque in the gears it desires so shifts around alot more than usual. Its not actually the gearbox that is at fault, but the engine! The symptoms you describe would point me towards this as a first point of call. Get the engine running well, (fresh oil, fresh filter, injector clean, intake clean, MAP, MAF etc etc, (whatever is relevant to your specific models engine health), once the engine is as good as it can get, you can go and run the gearbox reset routine in ECU Scan. I have written how to do it in other threads to have a search around.
These TF80-SC boxes are rock solid, so don't be so sure its a gearbox fault!