Before you go start ripping the engine apart, try a known good oil pressure gauge first. Second, the oil pressure is regulated by the relief valve spring within the oil pump. After these engines are ran hot mega times in a chassis that is heavier than this engine is designed, the spring that regulates the oil relief passage can become weak from the heat cycling. A quick cure is to remove the oil pump and shim the spring so it opens later and relieves oil at a higher pressure. Shimming is easy once the oil pump is accessed. You will see a cotter pin, press inside the hole to depress the spring and remove the cotter pin. Ease pressure off the spring and remove. Replace the spring if needed and stack a washer on top of the spring (next to the cotter pin end) depress the spring and spacer and replace the cotter pin. This will cure most fluctuation issues of the oil pressure variety unless the engine bearings are worn out, excessive piston clearance etc..
I hope this makes things a little clearer
~Gimpster~