Its a problem I had identified on a radiator before,I did everything I could to get the car to cool .The end result was found by taking the radiator down to a shop for service,they took the tanks off and found the radiator was unservicable .The job requires them to rod out the gunk that settles over 20 plus years in a long unserviced radiator ,they then put the tanks back on ,all for $85.They found a radiator out of their 1000 plus inventory that was good to go.