You miss your Duramax? You should be beaten for that comment!
Well, several things that I'd suggest if I were in your shoes.
1.) If you have a code reader, find out what codes are being thrown, and head in that direction to continue your diagnostics. Just because a "check engine light" is not illuminated, that doesn't mean that there aren't any trouble codes being set or that are pending. Idiot lights are just that... for idiots.
Let's face it, with any modern vehicle, you are 2/3rds screwed when you don't have the ability to plug into the OBDII connector with a factory computer scan tool. You'd be amazed what can be done with a Chrysler DRBIII or now their new Starscan scan tool. (Too bad they are insanely expensive and unless you own a shop, you just can't justify purchasing one of them.)
2.) If you can, without causing too many other headaches, remove the entire Edge power programmer crap and put it back to stock. I'm pretty sure that "65Fathead" also suggested this.
Start over this way to see if your Cummins runs/operates like a correct stock truck should. (Remember, a stock truck doesn't blow black smoke and won't launch you into the seat like you are trying to blow up your transmission, but it'll move right along just fine.)
Anyhow, if the truck resumes running properly after removing the Edge stuff, you've found your problem. Being 12 yrs. old, there's a good chance there is a bad wire or connection somewhere, particularly if it wasn't properly installed and if the wiring wasn't made to be "weatherproofed", so to speak.
To be honest, it sounds like you don't have a lot of experience with this stuff. No surprise, as most people don't until they are forced into it when a problem arises. Plus, there's nothing worse than taking on someone else's modified truck that has been all "hacked up", with splices and rat's-nest poorly installed wiring messes. UGH...
3.) Perhaps you have a diesel performance shop near you? If so, you may want to bite the bullet and take the truck to them to have them figure out what's wrong with it. I know taking the truck into a diesel performance shop is probably not what you want to hear... but in the long run; it will be less stressful, your truck will be fixed faster, and it will be ultimately cheaper than just throwing parts at the unknown problem.
4.) Heck, you might just something as simple as a bad fuel lift pump, leading to an inadequate amount of fuel getting to the injection pump. It happens.
Have you checked your fuel delivery flow and pressure going to the injection pump? Don't forget... continued running of your engine will ruin your injection pump in a very short time if your lift pump isn't doing its job properly.
Why diesel truck manufacturers don't put fuel pressure gauges and shut-down safety systems in place on vehicles to protect the expensive injection pumps is beyond me. I guess they can't sell parts if they don't break.
As other guys on here have also mentioned... there are a LOT of possibilities that could be wrong with your truck being low on power. You just have to spend A LOT of time on diagnosis, and eventually you'll find your problem. Welcome to the world of electronically controlled diesels... UGH!