[L Gebhardt]

Also, it cant hurt to set the apeture higher than f8 may well improve the flat field of focus

Well, this is one way to solve it. But going to a smaller f stop will cut down on sharpness. Aligning the enlarger is the way to go. Most lenses are at their sharpest one stop from wide open. If the enlarger is aligned there isn't much reason to stop down more, except maybe for light control.