auto box with a mind of its own?

2000 on W plate, 2litre LX auto 5 door hatch. engine seems fine, starts and runs ok. at speeds over 40 mph the engine revs rise a little as the gearbox changes down a gear or the may rise a lot as the gearbox seems to change down two or more gears. this is whilst driving at a constant speed. plenty of fluid in gearbox.