I have had this same type issue , it turned out the be the oil pickup tube, someone shortened the tube by cutting off some threads & it had a ledge step on the very last thread causing it to not seal up good when hot & it would suck air & oil. the hotter & thinner the oil got the worse the pressure flutter was. the tapered threads seal up at the very end of the tube in the block threads

Last edited by csk; 09/10/20 11:23 AM.
1968 Charger COLD A/C Hilborn EFI
512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim
2020 T/A 392 Stock 11.79 @ 114.5