I just joined the site looking for a solution to this problem myself. My old '90 f250 5.0 was doing the same thing (at about 90kmi) and progressivly got worse. One day I turned the aircond on and it died everytime. I unplugged the power to the aircond pump and found it fixed the problem. Did some troubleshooting but gave up and wired around the pressure switch that controls the aircond pump control circut. I'm guessing but I think there is a relay that is made when the aircond pressure switch closes (low pressure), it turns the idle up and pulls the air cond clutch in at the same time. The problem is that somehow the clutch is engaged without the idle going up to compansate for the load. Anyway it's related to the aircond.

My 92 f150 is just starting to do it now, 96kmi, exact same symptoms. Sure would like to know exactly what the fix is? any ideas people?

 

I had a similar problem happen to me about 1 1/2 years ago. Replaced the IAC valve on the side of the Throttle Body, and all was good. They IAC tends to get very clogged with carbon over time and either cleaning it or replacing it should help dramatically. It is what controls the idle by letting air bypass the butterfly plates in side the TB.
Try cleaing it with some TB/Carb cleaner.
Try that and see what happens.

Luke