360 oiling problem under load

Funny, I've had the pick up tube swinging back and forth, loose in the threads, and never lost oil psi as the op and TNT says.
I'll bet it is debris that was left in the oil pasages, improper cleaning before assembly. Honestly I have never heard of or had a cracked pick up tube, let alone would I ever miss something like that during inspection and assembly.
Dirt in the pump spring, anything over 2800-3000 and the psi will drop into the teens.
Best of luck finding, but make sure you really find the cause...That way people can learn something here.