Block value and Ardent defender surfing (why I was asking was the pally going really low) but given he is dropping hard thats not it.
To be honest my best guess is you have an issue with HOTS on the tanks. In guild we talk about V spike healing because essentially for the majority of stuff what you see is the tanks running along at 100% HP where the hots on the tank equal or are more than the low level background periodic damage and then what you get is a big V notch when the big hit comes. Then what we worry about is how fast that dent in HP is filled back up.
What you described sounds about right the big hits will be around 35-40% of your HP so for you to die the problem wasnt the hit itself it was the period before the hits in which you HP was not 100% and why was that.
So the question is why aren't the HOTS on you properly and it may be something like the pally is controlling adds or something better and the healers aren't having to move. I am pretty sure it's not gear related I think it's something you or the healers are or arent doing.
Sort of in my mind also is the fact the pally has made some very clever gear choices and interesting spec its not cookie cutter ... how do you rate his tank skill versus you? What I am getting at is some damage is avoidable by good tank movement .. that has nothing to do with gear.
** Remember Warcraft players fail in directions you never thought possible.