Your running out of oil in the sump at those RPM at the end of the run Been there done that with a stock 6 quart Hemi pan in my old pump gas Duster with the 518 C.I. 400 block motor if I let it get low on oil down to the add mark. I had the cam #4 journal grooved to oil the rocker full time, never saw that on others motors that didn't have that mod done to them
I ran 5W20 Wt in that motor and I worked on the six pack so it would idle at 850 RPM in gear just to show all the nay sayers that a big roller cam motor can idle at or below 900 RPM in gear with any carb set up That motor had between 18 to 20 Lbs. with oil temps above 180 F at or below 900 RPM I had cut the bypass spring so it had 65 Lbs. above 4500 RPM no matter the oil temps I ended up shifting it at or above 7000 RPM for hundreds of runs with no bad results either
Add the extra quart of oil to see if that fixes it or not If it does help either work on improving the oil return or add the Accusump.
I have a new 3 quart kit to use on my next fast car with a shallow oil pan
Please let us know your results