Cold idle when hot?

[Deadduck]

Another question I am having some oil pressure problems eg cold start or any start oil pressure is 4-10 psi till I rev up then might go to 30-40 psi then will drop back down really low sometimes so low oil light will come on. under load up a hill it was at about 50 then has bits where it will suddenly drop down to 20 then back up for about a second engine has had full rebuild oversize pistons ect oil pump was checked was good also all bearing clearances all perfect thinking air getting into where pickup goes to block or something like that all pressure reg was checked as well what can I do?