1993 W124 300D Almost dies when I let off accelerator
 

This happens especially if I have the A/C on but it will do it whether it's on or not.

Basically, if I am putting around the parking lot in first gear and I let off the accelerator, the car will shudder and almost die.

It won't do this if I'm at higher speeds and probably not in first gear.

Once, on a very hot day when the car was overheating a bit because one of my aux fans was locked up, it actually died and made a "squishing" sound as it did so.

Again, this only happens when I'm in first gear and I let off the accelerator abruptly.

It will stumble and shudder a bit before regaining an appropriate idle speed.

What gives?