chugging when coasting?

i own a focus zetec 1.6 facelift model 2001,when driving normally the cars fine but when i take my foot off accelerator and let car coast itself the car begins to chugg when it drops speed and feels as though it is going to stall,also my speedo is starting to move eratically when driving(showing that im doing 70 when im only doing 30)and the volume on my radio goes louder when this happens,ive been told it could be emissions valve but rather seek some second oppinions before changing parts i dont have to