as Said change the water pump for a metal impeller one, the standard ones are plastic and crack then just spin on the shaft resulting in the headgaskets failing, which is an awfull job to do,(partco sell the metal ones) I have a mk1 V6 which is mechanicly the same as an st24, they are quite quick mind you with loads of low down grunt.
also on the st24 or any mk2 V6 watch out for non working secondaries(they open over 3k rpm, this is electronicly controlled on mk2's and the box is between the 2 cylinder banks so gets hot and fails over time, about £150 to fix.
will never go back to a 4 cylinder mondy from now on, there that good!!
steve