01 cavalier coughing at a constant sped
 

2001cavalier,2.2L,170,000 miles. Started coughing/missing 3 weeks ago at a constant speed and now it's gotten real bad. Starts coughing at about 40 MPH but not as bad at 80 MPH. It stops coughing when accelerate, making it shift down. Like it's not getting enough fuel. It's an automatic. Replaced plugs,wires,air filter,fuel filter and ran some injector cleaner through tank. No check engine lights have come on. Had a mechanic hook a computer up and nothing showed up except something about injector band width. Think that's what he said. He said it went from a 10 to 5. Didn't understand what he meant. Any help would be appreciated.

Could be a possible fuel pump going out. Or could be repeated to engine timing. Even though it has a timing chain sometimes they do ware out. Check fuel pressure first. Then engine timing