The only thing that could be possibly happening is that you are hitting enrage. I believe you need to get styrak down to 15% before a 5th manifestation or you will hit enrage( it may be less, cant remember right now). But it seems that you guys are hitting enrage if it is doing that much damage. This fight is a tough dps check, but the next time you attempt him, take a look at styrak, he will have a enrage icon in his buff area if enraged.
That's what I was thinking but we have him down way before a 5th manifestation, usually just a touch after the 4th