04 JD 6620 with 6068HL engine PROBLEM HELP

[dabaka]

Hello folks I am new to the forum and I am writing down in behalf of a friend with that specific JD.
He appears to have a problem with what I beleive either fuel pump or the gas pedal
Fuel pump is Stanadyne with electronic control from the pedal.When it runs and heats the engine to normal temp it starts acting weird.It works just fine when its calm but once you get to the 1400rpm's and over you start hearing how the sound of the engine changes dramaticly,cant really describe the sounds tho,but its not top or bottom end related,cuz when the line to the 5th cylinder is loosen from the pump it softens that noise but still has it.Then if you try to accelerate the pedal does not react immediatly but it lacks a bit.Then once you shut down the tractor and it stays for i while eventho hot it may not do that specific problem for few hours.We've called JD dealership in Bulgaria(where i live) and they said they wont be able to check if its the pump or not no matter that they hook it up to a computer
Any suggestions or anyone heard something common to this problem

Thanks in advance