Diesel Bombers - engine miss issues

its a 99 2500 cummins w/ 160 000 miles. when it gets hot, the engine misses like crazy, but only when its hot, and only when its in park or neutral, or when it is coming down from running at speed in the rpm range that it is when its in park and neutral, like when I let off the throttle.

and another thing, it will only rev up to 2400 rpms when its sitting still. it only goes up to 2000 rpms MAX when I punch the gas. I don't know if that's normal or not.

I was told by a diesel shop that injectors 2 and 4 are weak, and that's all they know from the diagnostic machine. but I got a baaad vibe in that place. like a screw you over feeling. I don't trust them.

Heres what I did. I cracked all the lines going to the injectors, that connect up to the connector tubes, and they were all spraying fuel, so I know fuel is getting there. but I don't really know what im doing, and I don't know if that tells you anything.

I did replace the fuel pump, because the fuel pump was not cutting on at all, I could not hear it. and after I replaced it, I still cant hear it cut on. but I guess its turning on, because the engine is starting, right? and I was able to start it after I replaced the pump and the fuel filter, because the pump HAD to have primed the system, right?

I DONT KNOW, im just trying to fix it up to sell it, im sick of this thing, its a lemon. ive already sunk about 3 grand into this thing!

Help anyone???

---AutoMerged DoublePost---

I forgot to add one thing. when I put it in gear to drive, it does fine idling and accelerating. except when im slowing down from running at speed, when the rpm range is about where it would be if it were in park or neutral idling. it only idles rough in park when it gets hot. I was thinking it could be a cps, but if it were a cps, wouldn't it idle rough all the time??