Surging engine
 

I have a 2000 Ranger 2.5L with a 5spd. For the past couple months I've had a light surging that I can feel when holding a constant speed. It's especially noticable in lower gears. I've checked for any codes in the computer and it says there are non. I've also noticed it spark knocking a little at interstate speeds and a fair reduction in gas mileage and a little power reduction also. If anyone has any suggestionsit would be greatly appreciated. Thanks.