12.7 detroit problem, some help needed

Hello Got a problem with a 2002 12.7 detroit no egr. Last trip that i did while driving no matter uphill or downhill for a second was acting like was going out of the gear(so thought is the transmission) but was going back in gear, rpms fall down about 100, and then everything back on. Got transmission(super 10) out, changed sinchronizer, back big gear(inside teath look chipped), bearing, some other stuff. Front part of tranny was looking perfect, and everything we did was the back. Did the pick up next day and drove fine with no issues. Today at parking lot turned on the engine, put the cruise on for higher rpms, and for a second happened exactly the same thing like before(but i was idling now not driving), rpms went down about 100 and back on. Went outside to look at exhaust, and when it happened again saw black smoke. This engine does not smoke usually at all. Engine is got 1056000 miles. Did oil analysis, everything is showing good. No check engine light. Burns about a gallon of oil between oil change. Any other ideas? Thank you for help.

 

turbo changed 80k miles ago. If it has coolant in fuel it will smoke always isn't? How to check that? I am not a mechanic but i do lots of staff on my truck. I don't trust mechanics any more, at least here in chicago, they only think about making $ instead of solving mechanical issues.

 

if it's coolant, how the coolant can get into the diesel? Anything to do with injector/ injector cups? Where else should look?
Once the CEL is not on, it means it is not electronic problem, correct?

 

I will be going anywhere a good mechanic can be found. So where is he?