Before you spend money on a recaliberation unplug your maf and take it for a drive. A failing maf will make the gear change clunky believe it or not. And seeing as it only does it on the down change i would say it is more than likely the maf. Trust me i had the same problem on my sele. Its also free to check so no excuses to not do it. If it feels like you are dropping the clutch on a manual when down changing it is probably the maf. The sellespeed ecu relys on correct readings from the maf to perform a smooth gear change. If you dont know how to unplug the maf just do a search on the forum as it has been covered a thousand times. But please do this for your wallets sake.
You will get an engine management light on when it is disconnected but this will clear a short while after it has been re connected. The car will probably run like a dog also but you should see a difference in the gear change. And if you do. Then you know what the problem is.
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