series 60. air in coolant

hey guys in newb here, we have an engine that is just giving us a run for our money. in a span of 5-6 days the heat exchanger with have an air pocket that will drop or coolant psi low enough to set off an alarm. with in those days coolant will exchange between the heat exchanger and expansion tank and will at times foam up. it wont over flow the expansion tank unless we add every time its low. we have pressure tested the engine when its how and will hold air for about a full day. we have replaced the head, turbo, exh manifold, several caps and oil cooler. we are stumped. we get your normal exhaust manifold crack that pressurizes your system but it also darkens it. this coolant is still clean. have have not borescope the cylinders either incase there's a crack in the walls but i would figure we would see something under psi? maybe?