What can cause this
 

Truck in sig runs great, will pull anything I ask it to. Just seems that about 1/4 of the time it's sluggish down low especially, maybe just a bit down on power throughout the powerband. But sometimes (like today) it seems like a brand new truck, tons of power everywhere and very little lag. On the gauges everything is just about the same, although fuel pressure was dropping when hot over the last month or so. Fuel filter was new, so I stretched out the ofv. spring to get pressures back up to 35 on highway and 22-26 idle. Pressure drops about 2-3 psi at full throttle. When empty boost is always maxing out around 32psi, egts stay under 1000 unless I'm getting on it, even then it doesn't go over 1300 until about 2700rpm. Towing is different but still very managable for temps. Smoke is a visible haze at wot, little puff at first jab of the pedal.

If I get some time this week I'll check the shut down solenoid, has been a while since I cleaned it. AFC housing is stock, no fuel plate, stock (14*) timing, stock turbo and injectors. Mileage is good, pretty solid at 20mpg going easy, very rarely goes below 18 unless towing. Any other ideas? I was wondering if the AFC could be getting hung up some times or if it just needs the "fine" tuning.