Vauxhall Corsa rev problems as well as noise

I have had my Corsa for nearly a year now, and the last few weeks the engine has been getting worse. The revs get up to about 1500 then start bouncing around that aria, the only way I can get it over 1500 is if I put my foot down all the way, this means that when I’m driving I’m ever accelerating or slowing down. It is imposable to keep the car at a steady speed with revs above 1500. I have just had a new Exhausts fitted because the old one fell off. Don’t know if that will have anything to do with it?