Bought an 03 Carnival last week, K5 engine, 2.5ltr V6, 121000 klms (75000 miles), 5spd manual. Car sputters and runs rough in 2500 - 3500 rpm range in all gears. When I reset the computer it runs fine for about 10-15mins then goes rough again. Computer said it needed O2 sensors changed, (I've done that) Prob still happening, anyone had this problem, trying yo find a solution.