Compression is 260 p.s.i. ? That's on the bottom end of "acceptable". The beast is tired. Use a cetane additive and drive it until it quits. Then throw another engine in it.

4 Best Diesel Fuel Additives of 2018 (And Why They Work)

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now