The revs being held when on the move is correct, and shows at least that the wheel speed sensor is working.
The first thing you need to do is get a emissions check, as this will almost certainly tell you if you have a problem, it could still pass with a slightly dicky lambda, but if a mile out will soon be apparant.
A "oxygen sensor", or more commonly called a lambda sensor measures the oxygen content present in the exhaust, from this it determines the exhaust emissions as the oxygen content will change in relation to mixture etc.
It will be found in the exhaust near to the front with normally 4 wires going to it, easy to change if at fault, but you would have other symptoms if that bad.
tabetha