Posible causes of varying oil pressure
1. Loose clearance on bearings. This description of good oil pressure cold low pressure hot is a common problem in high mileage/ worn engines. Try 20W50 oil if it comes up you can go to heavier straight oil.
2. Pressure spring in oil pump poorly heat treated. looses tension as it heats up.
3. Where is your oil pressure gage attached? Try the attachment hole beside the distributor in the rear of the block. Your bypass spring in the oil filter mount may be partially bypassing the oil to the filter and thus to the lower oil port on the block. try high performance filter like a Wix or K&N and block off oil bypass for full oil flow through the filter.