very probable that you have internal crud on the tubes inside, jd makes a rad and cooling system flush that uses oxalic acid with water under load to dissolve the crap, then a flush and neutralizer . it works well, and or lots of old timers would drop the coolant and fill the system with water and some tide for 2 hours of work, drain and flush and refill with 50/50 coolant. most coolant has less than 5 years life, and when the additive package and inhibitors go the cooling system suffers like you describe. maintenance of cooling systems is often neglected. cummins has test strips in single packs that can tell you your additive status and ph, as well as coolant freeze point, use them, they also make an additive to replenish the coolant midstream of its life.