Let me first say that several mechanics have tried figure out what's wrong.
I'm a FedEx contractor, in Florida, and drive a 2002 Workhorse P440 work truck (chassis by GM). The problem is as follows.
a) The first and least disabling stage causes the truck to behave as if there's a governor on it. At about 40-45 mph, you can press the accelerator down further and nothing happens. You don't pick up speed, there's no change in the sound of the engine and the rpms don't rev. b) The second stage causes the truck to dip or dive when the accelerator is depressed too far. What I mean by that is, it behaves as if you were driving relatively fast and suddenly remove your foot from the accelerator. Actually, it's an exaggerated motion. You press the accelerator and the thing almost nose dives, followed by a slight sputter. c) The third stage causes the truck to behave as if it's running out of gas. You have to baby the gas pedal to get it to stop the sputtering. At this time, you can't drive much faster than 15-20 mph. d) The fourth stage: At this point the truck is definitely going to stall. It will restart but with the stage four behavior.
Usually, after the truck has been sitting for a while it will restart and run better, sometimes, much better. If it sits for 25-30 minutes, it may not have a problem for the rest of the workday.
There is one other thing. This has happened several times, but the last time it happened was several months ago. There have been times when the truck wouldn't start. When you turned the key nothing happened. It was as if the battery was dead. No dashboard lights, no clicking sound of a weak battery, nothing. The headlights would come on and the battery seemed to be fine. I had it looked at a few times, by a certified mechanic, but usually by the time I got the truck to the mechanic it was working fine and they couldn't troubleshoot it. Finally, I got someone to come to me when it wouldn't start and he found that a wiring harness, on the steering column was loose. He tightened the connection, with pliers, and the problem was solved. He also believed that this was the cause of the problem I've described above. Since, that mechanic did that, I haven't had the "no power dashboard" problem again.