In my experience players who foul early tend to (but not always) take it easier for the rest of the game. GS and your CT probably made your inside players tough to defend against early in the game but as time continued that changed. This was usually the case when I played Sharman: I used to hold leads after the 1st quarter or at least be in the game at half time. Then things would fall apart.
I have decent back-ups at C and PF so the fresh players coming in the fourth could stand up to your players who had been in the majority of the game.