Head scratcher
 

Ok here it is.....

2003 F250 6L Powestroke...
1. Intermittent rough running/stalling mostly at idle.
2. seems like condition occurs while in reverse and braking although has occurred while at idle in park.
2a. During the rough running, the speedometer starts jumping all over the place but then returns to normal (while parked.)
3. I commute to the desert area and live at beach....condition has only occurred while in the desert (temperatures over 110 degrees) not sure if that is related but it seems odd.
4. I recently installed a Viper alarm system with an auto start feature just before this problem started. I thought that might be the problem but I bypassed the alarm and the condition still occurred.
4a. Truck is equipped with a Banks six shooter but this was installed before i got it. I'm not even sure how to bypass this if it is a potential problem.
5. Problem does not happen all the time and not even every day and has not happened yet in normal temperatures, (50-90 Degrees) just in the extreme heat.
6. I did the FICM check and got 48V in both on and running configurations.
7. The injectors and glow plugs have all been replaced in last 10,000 miles.
8. My guess is it feels like something electronic, possibly breaking down in the extreme heat..... ECM? .... I'm not sure what to do next.:argh:
9. Last thing, when this is not occurring, the truck runs absolutely perfect! no issues at all.

any ideas?

Check the ICP connector for oil or excessive abrasion.

This seems to be a problem that pops up every now and then. I had the same problem awhile back and couldnt find it for the life of me. Took it to Ford and they couldnt find it either. So i started paying attention to it when it happened. Then Id tap the dash and speedo would bounch. kept following down and tapped the stearing column and the speedo bounced. But couldnt find the problem. It eventually quit for a week or 2 but started again. Where I left off at under the stearing comumn I moved wires or tappped the firewall. It eventually led me to the Fuse Box on the drivers side fender. The cab was off before I bought it for a head job. Found that the snap connectors for all the wires right under the fuse box were loose. Two of them snapped when I pushed on them and have not had a problem every since.
Every thing you discribed fit my problems exactly. Hope it helps.

Yeap... sounds electrical. Either chaffing in the harness somewhere or loose connections/grounds. try to clean everything up and take your time looking... crappy job.... but patience and a keen eye should find what you need to see.