I have a 5065e with about 1500 hours. It works fine pulling my brush hog for about an hour or two and then it starts to loose power. Not all at once. It will bog down going up a hill or through thick grass but by about 20 minutes after the first sign of a problem it can barely pull itself up a hill with no other load. The loss of power is always accompanied by a white smoke.
If I let it sit for an hour I can come back and get another 45 minutes of work before the 20 minute decline starts again. I changed the fuel filter and tried leaving the fuel cap loose. It worked better for a few hours and then back to the same problems again.
My neighbor, old fisherman type said he thinks it’s the supply pump. I’m guessing he means the lift pump but I don’t know Diesel engines at all so correct me if I’m wrong.