2001 fld 120 detroit d60 12.7 issue

Please help
2001 no egr 12.7 detroit is acting strange. When could it runs good but when it warms up it starts to jerk unless I hit the pedal all the way down.It gets worse when low on fuel.The fan started turning of and on for no apparent reason.When on cruise it has no problems accept for the fan sometimes comes on and off.
I took it to mechanics and they changed gasket on intake manifold which was bad and turbo boost sensor.Nothing changed.I took it to another mechanics.
He plugged in comp and found the reason for jerking was sudden loss off turbo boost.There were some problems with engine harness so fixed it but nothing again.Then he put another ecu which didn't produce any results.
Few weeks ago when I cranked it up it started blowing white smoke and puffed for a minute or two and then started working normaly.
the only code that is flashing is 62 and no check engine light.
Please help
Lenard

 

Injectors are fine no isues,fuel pump good,fuell filters new,engine harnes towards engine ok other one fixed.I am running out of ideas except one,surge valve, and he is going to check it tomorow.

 

all is good with respond time according to computer readings,he will check o rings tomorrow.As I said the only problem that the computer is showing is sudden loos of boost for short period of time

 

how much does the engine harness costs and where can you buy it