Should be pretty easy to diagnose. Have you looked for any telltale fur? Seriously, have you pulled apart the intake side of your A/C and looked? I know that they are famous for collecting leaves and other organic debris. I recently vacuumed out my A/C intake and was amazed by the amount of crud that had accumulated there.

Both my W140s and my former R170-based Crossfire collected amazing amounts of gunk!

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles