Unusual problem with an R1800...
Don't mean to bash MAC but did you recently update something?
Epson's MAC support is dismal. If you have a friend with a windows laptop that's willing to temporarily let you install windows R1800 drives you can test it to see if it is your printer or your MAC / Mac Epson driver.
If I understood your post correctly you are only getting the 5%cyan cast on the outside of the image or what would be the border. All the way to the edges? Or just to the printable edge of the paper. The only way I could purposely achieve that would be to layout the image over a document with a 5 cyan fill.
What you are describing is pretty bizarre. Any chance you can scan the print and post a pic of it?