There are only a few culprits to to these problems.
Fluctuations and loss of pressure:
1. Oil pressure fluctuations can be caused by a sticking or worn oil filter bypass. This bypass is located between the oil filter and oil filter adaptor. 1987-1991 engines did not have this bypass. To see if you have this bypass unscrew your filter and see if it has a oval washet behind the filter stud. If you do take the stud off and remove the pittle composite plunger (has three small tangs) and the spirng behind it and clean and re-assemble.
2. A loss of the lead or sludge on the oil sending unit or a short in the wire that connects it can lead to this issue. The sending unit is located on the passegner side in front behind the filter on the lower part of the engine block. A faulty sending unit will not cause fluctuation but a faulty gauge will. In CJ jeeps/AMC powered boats the gauges had PSI readings and in cars/AMC powered IH buses and travelalls and full size jeeps they used a dummy light for oil pressure
3. Loss of oil can occur if your low on oil or your pump needs surpass the volume of oil draining back to the pan (very common). AMC V8s like to trap oil in the tops of the heads and in the front of the timing cover. To combat this extra capacity oil pans are available but not all are made equal. The better pans allow oil to flow back to the sump area minimizing stops between block bottom and pan bottom.
4. A lack of startup pressure is usually a symptom of a sticky/stuck or missing oil pressure bypass plunger or spring. AMC used 356 cast aluminum for the oil filter adaptor material. This is much softer than the steel pressure bypass spring and can wear in the bore holding up the plunger. You can also note a sudden 65 psi and then 0. This happens when your pressure spikes to the maximum the factory pressure bypass spring will allow (65-70psi) shrinks and allows the plunger to move back and stick in a groove worn by standard service. In normal operation the plunger will move forward and back frequently as the oil temperature, rpm and contaminants reach the oil filter. The pressure bypass is the systems first pressure valve helping against blowing the oil filter off its mount. This bypass is critical and should never be deleted. It is located on the side of the oil filter adaptor facing the oil pan on all 67-91 motors (except Pacer V-8 ) and has a large 7/8 bolt and aluminum or copper washer for retaining the pressure spring and plunger.
5. Oil filter types also come into play. Some will stress the quality issues about certain oil filters we wont get into this we will talk flow. Fram and Fram duraguard filters for the AMC V8 filter at a very small micron and subsequently require more media to allow the same flow as Wix/K&N/Napa gold filters for the AMC V8 67-86 and the metric filter AMC V8 87-91. This means you will see a larger oil pressure with the Wix/K&N/Napa gold filters than you will with the Fram and Fram duraguard. Because AMC V8 engines have a pressure bypass before the filter excess pressure from a lower flowing filter is relieved by sending the oil back to the pump. This is normal operation on all filter especially when the engine is at sub zero temperatures or if the engine has very high viscosity lubrication in the crank case.
6. Bearing clearances and general motor clearances are critical to good oil pressure. AMC V8s use oil paths that are drilled into the block after casting. The oil path runs to the lifters first and then to the main journals while feeding the camshaft along the way. This means if you have a lifter that collapses and kicks and pushrod to the side bending it the lobe could possibly push the lifter out of its bore but most likey will cause massive bleedoff of critical oil flow for the camshaft and main/rod journals. If you suddenly loose oil pressure and hear a mess of noise it may not be a main or rod bearing that has spun (lets hope it isnt) rather it could be a bad lifter.