sounds like you need to space the middle ring away from the crank spider tab to get it equidistant between the outer & granny ring
these spacers do exist, but can be buggas to get hold of.
M953 (9spd) XTR chainsets came with them, the only difference between M950 (8spd)
i've got a small jiffy bag full of em somewhere... just finding em :?
build the chainset up with the rings in place, but leave the middle-big ring bolts loose, push the big ring against its spider tabs, then slowly move the middlering away from the big towards the granny ring until its centered, then guesstimate the size u need.