CHRISTO!
I have some good advice for you, since i have the same problem regarding throat / coughing, even with a vaporizer! I find that my throat is very sensitive to spice, and the more I try to take, the more it feels like it's "closing up" and rejecting it!
HERE IS THE SOLUTION...
go to your nearest pharmacy, and buy some "SORE THROAT SPRAY" make sure it says "anesthetic" on the label (should contain Phenol). Spray this on your throat, gargle it a bit, wait a few minutes, NOW try taking dmt hits!
I was SOOOOO surprised when I loaded a huge bowl, and was able to clear it in ONE HIT without any discomfort, something I could never have imagined doing before getting the spray!
However, I'm in the same boat as you, I haven't been able to breakthrough for some reason, not sure about my body. I may have tolerance to it. Latest session started out with 30mg - nothing intense, felt happy, carpet was alive. Then did 50mg soon after, slight colourful OEV patterns in the room about 50% opacity, no CEV's - then about 10-20 minutes after that, did 80mg, no effects whatsoever. I was sitting there and looked over to my friend and said ... "Wow...i'm still here, was kind of expecting SOMETHING with that huge of a dose..."
Pretty weird, might take a break from it all for a while. Probably need to START OFF with 80mg dose.