By no means an expert but after just going thru an install of my own I know what you mean about using the drop calculators and all. Not sure how far you had to run but if it was all run with 10ga and within 300ft I would have thought you were fine. After reading this I'm wondering a couple things.
Definitely check it with a tester for power. Rule that out first.
If the pump runs fine free flow but then acts up when hooked to air line and diffusers, I'm just wondering if there's not a collapsed fitting somewhere or a kink in the air line(which seems impossible I know but strange but true stuff can happen.
If that still doesn't help, check that pop off valve and replace or bypass it temporarily till you can cross it off the list of possibilities.
Sorry not much new advice but those are the things that I would narrow down first.
GOOD LUCK!!!!
