Hesitation mostly between 38-50 MPH

1997 Dodge Caravan, 4 Cyl. 150,000 miles. Van hesitates/sputters when accelerating from a coast. Mostly when accelerating (like going up a hill) at 38 mph through 50 mph. I changed plugs, gas filter (at the tank, which needed to be done anyway). Still gets decent gas mileage. Doesn't blow smoke. Doesn't stall or overheat. Thinking O2 sensor, which was replaced about 60,000 miles(approx. 3 1/2 yrs.) Ago. Or possible vacuum leak? Starts every time, no problem. Does it no matter what the weather or driving conditions are. Any ideas? Also what is the procedure with turning the ignition on/off to get the codes? How do I do it? I appreciate your wisdom on this.