Hi again!
If switching cyan and black, switching it back (printing) and then switching black and yellow (printing) did nothing then the drums should be fine.
The IBT belt could be whats giving you the lines, sure. You'll usually see if it's damaged when taking it out. (Is it the exact same measurements between the lines everytime?) can you replicate the lines issue and get a clearer image of it somehow?
And yes, there is a transfer roller in this machine 
(you can find it by looking through THIS, close to the fuser)
It could also be a developer housing issue or a bad drum contact in the magenta drum area, as it only affects magenta.
Try taking out the magenta drum and another drum, putting em in a dark place , looking to see if there's any difference between the two contacts in the back of the machine.
I know "Fiery" is a developer-expert, I'm gonna see if I can get a hold of him. He might have some more suggestions.
Best Regards
Kimzi
--------------------------------------------
Please remember to select "Accept as Solution" for posts that helped you solve the issue.