Automatic gearbox behaving strange

Hi,



My automatic gearbox (75, 2.0 cdt) sometimes changes a gear down, increasing to 3000rpm, most often on small hills.



It is very noticeable when you have driven the car for years, and know exactly when it usually changes gear.



Any idea what would cause that ?



(gearbox oil was changed 2 years, and not many miles, ago (correct oil) )



Thanks,

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