Rough idle

The car runs fine when continuous driving but when you come to a stop it idles rough. I could put it in neutral and it would get better once I put it back in drive it idled rough again as well as rough idle in park. Sometimes when I would slow down really slow coming up on a red light or stop sign and I didn't stop real fast if I just came to a complete stop slowly it wasn't such a bad idle but when I would come to a stop like normaly that's when it would idle rough again I have. Replaced map sensor, crank shaft sensor, sometimes you can smell the fuel inside the car like its not burning the fuel, but most of the time you can't smell the fuel. When I disconnect the battery for 30 minutes or so and reconnect the car seems to run pretty good for about 20 minutes then back to the same rough idle. When the previous owner took the car to be inspected before I bought it the inspection paper said that the car was a mittstubishi and not a stratus, the car has been wrecked and I believe most of the front end was replaced but not the front fender wells or inner fender wells so when it rains water goes up into the engine area, the battery area and where ever it can. If the fenders are missing could this be a computer issue? My boyfriend says that the engine doesn't go with the computer on the car when it was wrecked it could have been replaced with another type if engine that's why the inspection print out said it was a Mitsubishi. I guess also it seems to not be as bad of idle when the gas tank is fuller than when it gets down to a 1/4 if a tank