Just had emissions run on my 1988 f-250 with a 460 fuel injected. passed all but the CO % ran 0.01 on idle test at 740rpm which was fine,but failed at 2409 rpm with a reading of 4.30 is this a fuel problem when its on the CO side, runs good for 10 or fifteen minutes then when comes up to temp runs rough with no power. Any suggestions what I should look at.