I'm having the same exact problem. I have replaced the transfer roller but that did not fix it.
I only have this problem on paper used in the bypass tray. Changing the bypass paper type to Cardstock does help a little bit, but does not fix the problem.
Did you ever figure out what the issue was? Anyone else able to answer?
The prints from my Phaser 6510 have smeared color on the prints and also on the unprinted back of the prints.
I have performed Engine Cleaning, but that didn't help.