Heavy judder on Peugeot 206 1.4 16v. 2004 model?

I have a problem with my 2004 Peugeot 206 1.4 16v. It judders quite a bit between the speeds of 40mph to 50mph and the gear stick shakes. After this it mainly settles down through the higher speeds but can still be felt to a degree. It only judders when I apply the accelerator and is worse when you carry on accelerating to a higher rev before changing gear. If you change gear when the revs are lower it doesn't get to a heavy judder, just a light judder, but is still quite noticeable. I have taken the car to a mechanic and he changed the wheels from back to front but it has made no difference. There is no noise that accompanies the judder. I mentioned to the mechanic it could be a loose engine bracket or the drive shaft but he didn't think this could be the case. I recently got a new exhaust as the mechanic thought that this may have something to do with it and it did need changing, but it did not solve the problem. I had one of the fuel injector changed about six months ago as the "air pollution" warning was coming up and the car was stalling intermittently. I then had a software update and some new wiring done a few weeks ago as it felt a bit chuggy whilst ticking over and we had the "air pollution" warning coming up again intermittently. That has now stopped. It is still not ticking over brilliantly, but if you turn the air con off it seem to help. These recent problems might not have anything to do with the judder, but I thought it best to mention them. Any ideas would be gratefully received. Thanks! Paul.