Since I contributed to the original thread on this subject I feel I should maybe jump in here.
Instead of looking for reasons FOR what happened, let's look AT what happened. IOW at the evidence.
Memory OK to 50, then from 90. Not too clear back on surface.
We can discount Narcosis. It does not happen intermittently on the same dive, and the signs decrease with ascent.
CO2 can, with the right set of circumstances, begin to close your brain down. But it is subject to higher partial pressures (depth) and increased exertion/shallow, rapid breathing - failure to flush the gas from system.
Stress was not involved according to the poster, so we can discount CO2 as well.
The air quality in the diver's bottle, which was my original posting, could have had a bearing, esp. with toluene - as mentioned by COVCI. But would it's effects wear off with increasing depth? Doubtful.
This problem seems to have something to do with the descent. A couple of questions -
Did you descend in a feet down/head down/horizontal position?
All the way, or for which part(s)?
Did you have your mask flood at any stage during the descent?
Depending on your answers, a number of further considerations will be discounted, or enter the debate for probable cause.
Seadeuce