I am active C 5/6 quad and have the same problem. I determined that's it's related to pressure on my rear end, which is normally sitting on a well-regulated roho cushion. My longest trip to Hong Kong (from the east coast of the U.S.) resulted in a pressure sore and was extremely uncomfortable the way there (flew coach); and went wonderfully the way back because I upgraded to business class and could fully recline my seat. I've tried the roho on a plane but the high altitude air pressure messes it up. I've tried other cushions on planes but none is as affective. The only thing that seems to works, at least temporarily is relieving the pressure by leaning forward or sideways (if you've a friendly seatmate) or as I mentioned biting the bullet and shelling out the dough for first class or business. BTW- for regular flight first class seats work better but again cost.
hope this helps,