my coworker's daughter used to have 2005 Passat with "sealed" transmission.
around 115K miles she gave it as a "gift" to her dad as transmission was sticking on low gears and developed some vibration. car had a great body, quite worn suspension, but great clean interior, so he figured he would repair and keep it
upon getting transmission pan removed, he found some gunk, but other than that, he made a research and found that VW solenoids controlling valves are "open type" and get their magnets to grow "beards" of metal particles, eventually making valves to stick and transmission to develop "sticky gears"
he found that these solenoids are sold only in full pack, at some ridiculous price, so he cleaned it as much as he could, get fresh ATF if and traded car