After Preflighting a PDF, using Convert to CMYK, Flatten Transparency and Prepress Profile Convert to CMYK only the resultant PDF has a grubby halo along the edge of some white type sitting on an image. The type is part of the image.

I am using a 27" iMac 3.2 GHz Intel Core 5, 8 GB Memory, running Yosemite 10.10.1. 

The version of Acrobat that I am using is: Acrobat XI Version 11.0.10

After Preflighting a PDF, using Convert to CMYK, Flatten Transparency (high resolution) and Prepress Profile "Convert to CMYK only" the resultant PDF has a grubby halo along the edge of some white type sitting on an image. The type is part of the image which is 300 dpi.

It is like the image isn't really 300 dpi but has been artificially boosted to that to avoid being tagged by Preflighting,but when Preflighting the file it knows the original resolution.

I have screen grabs which illustrate the problem perfectly but do not know how to post them, if indeed they can be.

Any help or comments gratefully received.