Cat 963 21Z radiator

Machine has been creeping up in temp when working under load. No bubbles in tank, replaced the themostat and flushed the rad but still gets hot. When pouring water into the rad it seems to take a long while to trickle down. A new core is $3.5K plus fitting. Any ideas on cause and what methods are best to try and clear the score within removing it.

Cat 963 21Z radiator