Cenrury detroit 14l - chocking and bad mpg /need help/

Hi,
I've just buy my 05 century 14l detroit engine with recently changed head due to mixing oil into the antifreeze.
Truck looks good, engine running good, mechanics says it's good but when I'm loaded and accelerate is choking at 13-1400 rpm. It's like cutting the power for a seccond and then continue to accelerate, the weird thing is it doesn't doing it when I'm bobtail only or rising the rpms on place and when or accelerate by cruise control.
I thought it's the trottle sensor but it shouldn't be because it doing it only while im loaded.
It makes 5.8mpg at 13500 rpms with 60 mph which is another thing I'm worried about.

 

750k on the engine. What is vpod and how much does it cost to repair?

 

Yes bro! You gave me a priceless advice and I apreciate it! I've tryed to text you back but the system didn't let me becouse I'm new member. Just wonted to make sure I'm on the right way solving the problem.

 

I hope you are right. In just a few days I'll know for sure and will post updates.

 

I changed the fuel filter and notice a lot better fuel milage.
25k in te box 60mph ~1400rpm made 6.8mpg
10k whith same spee 7.14mpg
Today rying the same 10k lbs w/o egr and mede 7.47mpg
Turning it off the engine stopped coughing, incresed the mpg and noticed a lot better throttle response, and little higher oil pressure. It's all an experiment and give u the results.
next step is tune up and high flow muffler when I have a chanse.