My 124 does that a bit. It cools marginally when first starting the AC while stopped at idle. The aux fans cycle on and off by the temp sensor on the receiver/dryer. Start moving with RPM's over about 1500 and it starts cooling nicely after a while.
It was last recharged with dyed R12 just before I got the car in 2005, and I think it might have lost a little since then. The MB dealer that did it said it was low when they serviced the system, so there may be a very slow leak in there for it to still be working acceptably two and a half years later. We shall see how it fares next summer
...sure hope it's not the evaporator
