I have a small block with a m55a melling oil pump. It's standard volume high pressure. At 5000+ rpm the pressure fluctuates between low 80s to low 70s. Never drops below 70. According to melling the pressure release valve on the pump is set at 78 psi. It's the fluctuations from this valve opening and closing. Or is it starving for oil.
go thru the drain plug opening in the pan.. see if you can verify exactly how far the pickup tube screen is off the bottom of the pan..if its TOO CLOSE it can cut off flow at high speeds.if it's too far.. it can loose suction at high speeds...what.. loose suction...yep.. the oil trapped under the pickup screen area is being pulled in equally around and there is a LOW pressure area again under the screened area..if the screen is too high. at some speeds when the oil slinging off the crank and the reduced oil level can cause the pickup to loose suction..this is kinda a dangerous thing.. add an additional half quart to a full quart above what you are running now.. this will raise the oil level left in the pan at high engine speeds and hopefully keep the pickup screen covered deeper in oil..you also did not say which pickup tube/screen you are running... melling on their pumps uses wire screens to protect the oil pump from debris..some pickup manufacturers have used perforated metal for a screen.. and its FLAT on the bottom.. no area to build suction/low pressure in.. and there is an issue..the perforated metal... is not designed for this application.. perforated metal has MUCH LESS OPEN area for oil to flow.. than a similar sized opening wire screen.if you think i have it wrong.. just visit a 7-11 for a slurpee.. look at the end of the straw.. you will note that its has an open area.. for scooping it up.. but when you properly position the scoop area... you can pull a much larger volume of frosty liquid up for a more intense brain freeze.. the screen also has to be an equal height off the bottom of the pan.. for it to properly work at high speeds.. over on another forum here.. the pickup screen was at a significant angle off the pan.. one edge was close.. but the other side was probably close to an inch... you should have seen the bearings and his crank.. wiped.. from lack of oil flow...if you are getting intermittent readings... BE CAREFUL ... bearing damage happens really quickly...and since you have experienced oil pressure issues.. next time you have a chance.. you might want to examine the engine bearings.. or at least cut the oil filter open and check carefully for signs of bearing materials and steel or iron metals in the drained oil... a really good reason to clean the drain pan before each oil change..save a sample of it into the used oil bottles... date it and stick it on a shelf..this way you might have samples to have checked at a later date.. if you have a really big variable speed drill.. you can test the pressure cut off of the oil pump with a priming tool... 500 oil pump rpms is 1000 crank rpms.. if you had the oil pump off.. you could bolt it to a spare rear main cap off a blown small block.. big block or even 4.3 v6.. bolt that to some bar stock and hang it across a bucket with some clean oil.. a pressure gauge in a tapped hole in your bar stock..