Loosing low end torque when hot

[TowX]

This doesn't happen all the time, and there are never any codes stored.....

When the engine is cold, the 4.4 works just fine. Plenty of power and throttle response. But occasionally after the engine is up to temp I'll loose throttle response and torque. Especially noticable when pulling my trailer. I can run along at 70 with the trailer behind me, about 3,000 rpm, and have some throttle available to pull inclines. Other times at the same speed there will be absolutely no throttle response when I tip into the pedal.

On my 528 when I had the same problem I needed to replace the vanos seals but on that car it threw codes because the mixture was out of spec. No codes on this.

Thoughts??