My 2g sabb diesel is overheating

[Pete72]

My 2g 22hp sabb diesel is 30 years onboard with raw water cooling and has been wanting to overheat when run over 750 rpm. I reamed out the water jackets with a 1x19 wire and it helped some but not much. Would pulling the cylinder sleeves and sand blasting them or replacing the cylinder sleeves be the rite thing to do. Thanks for any helpful info. Pete Johnson. Aboard. Eliza Nelson