Regardless, looking back on it 158F is pretty high by convention. I'm surprised you've hit 1.018 at all. So probably you really weren't that high due to the imbalances in a direct fired mashing process. Convection and mixing etc...
Try batch sparging at a single infusion temp- say 153F...?
Agreed...158* is high.
Shoot for low 150s, if you can't get precise control of your burner.
That will make all the difference in the world.