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I was just watching a YouTube video where a couple of chaps (always wanted to say that) purchased a Boxster that had an automatic transmission, it worked good in manual but slipped between gears in auto, they changed the transmission fluid and filter and it worked perfectly. Its a long video, but anyone wanting to watch it can run it forward. The transmission work begins at about 11:45. Good Luck!
https://www.youtube.com/watch?v=oS_yKQRcln0
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