Detroit 12.7 DDEC 4 Interesting problem

http://www.dieselenginetrader.com/diesel_talk/categories.cfm?catid=9
You should post over here too, I haven't been on there for a while but they used to have some pretty sharp detroit experts.

 

Do you have a pyro/EGT gauge?

 

So you're getting air then, and if you were constantly losing 8-10 psi, you'd hear that for sure. If you just weren't pulling enough air, you're pyro would be higher. I'm guessing bad fuel lines as the next culprit. Do they appear original?

 

I have a '96 9400 12.7 with same issue. Have you found the fix yet? I still looking.

 

most likely electric . A freind of mine with EU truck ( mb actross) had a lot of trouble in what seem similar to your case,spent tons of $ changing mechanical parts . anyhow he found a sensor near the top of the engine near the turbo that was bad but went un-noticed to pc scan tool or dash. Warning

 

DO make sure you check ALL the parameters in the ECM. Just because the HP setting was flashed doesn't mean anything else was changed from prior settings.
While this sounds like a harness problem I would check to see if you're getting any air in the fuel lines.