2003 volvo isx15 cm570 issues

[Magnat1976]

Hello People,it's been a while,how you doing?
I like to ask your opinion..I been working and spending a lot of money on my truck...I like my old truck and miss how it ran before..
If I haven't spend so much money I would change to newer one,but I also don't like modern trucks...with Def and cheap machinery.
What I've done recently all suspension, tires everything in top shape..
On engine..now its has 860 k.300k after cummins dealer overhaul,I bought just after overhaul,cummins did everything everything except injectors...I have paperwork..
I replaced all sensors,all fuel solenoids (brand new),all fuel check valves,engine harness...and truck still acting really weird..intermittent...power loss,vibrations,sometimes more blow by,more smoke...
I was about to replace injectors,turbo..but this week,I discovered...when I disconnect power cable from batteries truck runs differently most likely as it use to be..softer,smoother and powerful..but for a day or two..
I suspect it could be engine ECM...but absolutely no fault codes...
Is it possible that Emc,software, hardware malfunctioning, with no warnings?I imagine it's 21 years old
I am about to buy the oem ecm,for my truck only rx ,no new ecm available and it's 2500$...+ plus programing..been thinking to look for used one,but who knows,it can be same
Please share your experience and thoughts. appreciate in advance