Cummins problems HELP!!!

[MandBtrucking]

Ok so I got a 870 isx in my 05 Peterbilt little back story I bought this truck back in October now I have never been around a Cummins I was a cat man and decided I wanted something different ok so this thing has 100k on overhaul and injectors now the lift pump is hooked into a toggle switch (I don’t know why it was like that when I bought it) egr is unplugged now when you start it cold the thing sounds like it is going to fall apart popping and missing banging and clanging was told by a few people that is how they are. Once it warms up runs okish it has a miss a miss so bad that I have been chasing ujoints and ride hieght because it gets this buck going on at higher speeds like 60-65 and only at neutral throttle throw it out of gear and it quits didn’t really bug me to much but been keeping track of my fuel mileage here lately and it is clear down to 4.5 I pull a pig pot but most the time I am loaded with 3000 pounds of baby pigs I think it needs to do better and it had just gradually went down now I can run down the big road for a 170 miles get off the ramp and I have a dead throttle for a split second I let it idle for an hour load and go to take off it acts like it is running out of fuel then boom takes off again I let it idle for 10 minutes it doesn’t run smooth at all until I get out on the road and take off no loss of power in my opinion just runs like crap I can feel it in the throttle and seat I took injector actuators off on Wednesday took apart and cleaned them they didn’t look that dirty to me put them back in same location didn’t have time to do timing but got it goin and it ran about the same as it always has the now today went out and fired it up didn’t really pay attention to how it sounded let it warm up and took off about 20 miles from home I noticed it IT WAS RUNNING AMAZING!!!! No missing not rough or anything I didn’t get to excited because my luck it would just go back to the way it was once I stopped got to the barn to load idled it up I always do loaded took about an hour went to take off and it didn’t attempt to die like normal so I ran it over 600 miles ran amazing nice smooth stopped to get fuel checked my mileage and I was up to 5.2 pulled out and it started missing again and did it all the way home bout 120 miles now I don’t know if this has anything to do with it or not but my jakes run good and strong until about 1200 rpm then it is like you start shutting cylinders off it never did that before... now I know I need a overhead run I don’t like shops working on my stuff to much in my opinion why pay for something that I can do and will have to do in the end now if it was a cat I would jump right in it but beings how i am new to the world of Cummins I need some help here what do you guys recommend??? P.S. after talking to some guys that knew the guy that owned my truck prior sounds like he has been chasing this problem for awhile but i think he just gave up any help would be appreciated!!!!