I have a similar problem with my 83 300d (225k). The AC blows cool, and then the air completely shuts off (no air blows). Within 3 minutes, it turns on again and blows cool. This continues to happen over and over and begins before the car is cool i.e. it isn't turning off to keep the temp at a normal level. If I use the middle or low speed on the fan, it keeps running. This only happens when the fan is on high. Ideas? As a sidenote, this car does not have the switch/clutch that turns off the air when acclerating, although when leaving a stop light I wish it did.

To the post above, I also have minimal flow through my middle vents. Those never have worked very well.