I have a 3126B CAT in a FL50. After about 4 hours of running it is losing power/speed, will drop from 65mph gradually down to 25mph. Changed fuel filters, air filter and oil. Still happened so changed the fuel pressure regulator. Still happening. No fuel in the oil, no fuel in the coolant, no coolant in the oil and no oil in the coolant.
What else can it be as to why is runs fine for 4 hours then has issues?