engine trouble, oil pressure diving

Mechanical or electric gauge?
Auto or manual trans?

First, I'd check oil level, then filter.
Next, I'd pull the valve covers and check the rockers for preload/lash and look for signs of abnormal wear.
Then I'd probably pull the intake to check the lifters.

The only things that cause sudden drops in pressure are low level, a bad gauge, or some sort of blockage.

It's also possible for lifters to cause oiling issues, and could be anything from a lifter that fell apart to one which is about to fall out of the bore..

It's also possible for the crank or cam to contribute to pressure loss, but these are rare.