One problem I had with my 1990 W124 was vacuum leaks inside one of the vacuum elements behind the glove box. The leak prevented the doors that direct air flow to the vents from working properly thus I never could get good cold air out of the system. I spent a lot of time messing with the freon charge (which was fine all along) before I located the vacuum leak.

But start with the aux fans up front, make sure the come one after the engine warms up.