91 lesabre engine surges when stopped

My leSabre surges when stopped and my gas mileage has declined. The service engine soon light also stays on. I replaced the oxygen sensor and that helped for about a week. I had it diagnosed and the mechanic said that the camshaft, crank and oxygen sensors kept coming up. he suggested i start replacing them. Is there a larger problem that could cause these sensors to go bad since I just replaced the o2 sensor and it is now bad again after such a short time? Also, from what I have read faulty cam and crank sensors would cause the car to stall and not start at all. My car runs fine on the highway, it surges when stopped and my fuel economy is bad. Looking for some insight before i start replacing sensor after sensor. Thank you.