Just a couple things come to mind from my experience:
The Evod has a Polycarbonate Tank which means its very vulnerable to acid flavors causing the plastic to fog or melt which can lead to a clogged breathing tube and other issues as well. Heres an article that might help: http://www.advancednictech.com/Articles.asp?ID=254
The coils are manufactured all over china from diff. companies and can be flawed, even a large pack can have issues. I have had a pack of five coils all been mismanufactured due to air gap at the bottom contact point of the coil not being cut which can cause an effect of almost no breathable passages.
A couple things to try, once your tank is empty and/or your ready to switch coils take off the reservoir and attach the bottom piece with the coil back on your mod/battery device (don't fire the device) trying sucking through the small tube on top of the fresh coil and see if the air flow is any better, if it is the issue is in the reservoir breathing tube. if the air passage is still blocked with a fresh coil check the contact point on your battery for flaws and inspect the breathing holes on the bottom collar and the threads on the battery device.
When o-rings go bad you get one or both issues, Leaking of the juice into the contact of the battery or around the outside if the body or Raw juice can flow into the breathing tube usually causing restricted or gurgling of the air passage. if the juice is free flowing into the air passage then the center tube on the resevoir is most likely flawed.
To be perfectly honest, at the price of the Evod, I would probably just replace it.
Hope this helps!