Automatic Corsa reving?

Have 51 plate auto Corsa (the tritonic (think thats how you spell it) gear box) and when driving down road, goes over roughly 40mph and rev counter goes into the red as if problem with gears. Then remedies itself but again will start again, few days back thought car was sluggish now this. Plan to take it to garage to hopefully tommorrow but any ideas what problem is?