rough running fault

Hi, I recently purchased a 2005 Accord 2.4 Auto petrol with a mileage of 130000 miles. Having covered some 500 miles in it I have found one problem. Between 1800 and 2100 rpm in 5th gear (55-60ish mph) the engine runs rough and lumpy, almost a feeling that it wants to stall. The revs don't drop...