Oil pump pressure relief valve...maybe got some dirt/plastic from chain guides into the valve causing it to stick. Not sure where it is on the 103 motors..but familiar with the ones on the 2.3-16V.

Bad crank bearings (spun) is one of the most common problems with dropping oil pressure. However usually when this happens it will give you an indication...pressure will fluctuate..when you have pressure..The needle will flicker regularily as the bearing spins on the crank. This pressure flickering is what let's you know it's the crank.