Need help troubleshooting 05' GMC Topkick 4500

We have a 05' GMC Topkick 4500 that has been in to be serviced 4
different times with 2 different medium/heavy duty truck service
centers.
This truck derates while on the highway causing engine failure where
the vehicle decelerates until 20-30 MPH. It will then begin to
accelerate, generate speed and downshift normally. I pull a 8-horse
trailer and normally have to pull over, let the truck decelerate and
then start over. At times the check engine light will come on, but
clears after being restarted 3-4 times. Both repair companies are
unable to diagnose, and the only code that has come up on the computer
is a low-fuel pressure sensor. We did replace the accelerator pedal
with the thoughts that perhaps the sensors in the pedal were
defective, however this was not the fix.
Any advice or someone with a similar issue and repair idea would be
extremely helpful.
Dave
.