98 gmc Z71 running bad

I have a 98 GMC Z71 pickup that has 115,000 miles on it and recently in the past couple of months has been running very bad. I don't know what it is exactly doing but it acts like it is running out of gas when driving normal down the road. When I say normal I mean driving softly not in a hurry. Now when I drive it hard and keep steady pressure on the gas it runs fine. I have change spark plugs and fuel filter. My check engine light has been on for several months before all of this has happened. I checked the codes once for this and it was my o2 sensors. I have not changed them but have changed catalytic converter around 3 years ago. Don't know what else to do right know your help would be greatly appreciated.