I do not think it is a memory issue.  I have played a 3 person map with 2 AI opponents set to normal difficulty and that game had a lot more units.  But I have tried multiple times to play a single player game against an AI set to challenging difficulty and it freezes.  I wouldn't even say I get to mid game, I get to where all the nodes are claimed and there are a couple battle fronts but only at the point where I have produced maybe my 3rd dreadnought so not really all that far into the game yet.  I have played this same map against an AI on easy and finished it.  I will try the other 2 player map and see what happens.  The map I have been playing is called Deneb.