Loss of power

[RFTLTS]

Hi I have a 99 250SD, 6-spd no Mod's but a K&N filter. My problem is that while pulling my trailer (~8klbs) my truck suddenly developed a significant loss of power. The engine sound has changed off idle with no load, very flat sounding. The power loss is from idle to >2200Rpm. The turbo effect is minimum but I can hear the turbo spooling. CEL faintly flashes intermittantly while going up hills. What power is left is very smooth. No smoke, and I have no scanner. Any thoughts. Thanks, Loren