It could be a number of things, start with the easy one others have suggested. It could so be your main and rod bearings on the crank or the cam bearings. Usually that's the case when you start up and you get high pressure cause the oil is not as thin as it is when it was a cold motor, then as the motor warms and the oil thins a bit and you drop pressure.
build thread: http://www.jeepforum.com/forum/f22/skyj-eccentric-build-1467426/ '90 YJ 4.2 with 4.0 head, HEI. AX15, NP231 SYE, Dana 30 4.56 open/ 8.8 4.56 detriot locker. 1" shackles
2" BL, 4" stretch all on saggy waggy springs.