2002 Chevy Impala hesitation

My check engine light came on, and I took it to be put on diagnostic machine and it pulled up front oxygen sensor code. The car was already hesitating a little when I first get in it to drive off in the morning, but the hesitating got worse after I took it to have the oxygen sensor changed. The converter was checked and cleaned. It slows down and speeds up again and again and have black smoke coming from the exhaust and a smell. They told me it was in my fuel line, I dont know what to do. Its like they made things worse after changing the oxygen sensor. I only had the car a year and haven't had it tuned up. Just changed the oil every 3,000 miles.