Cosworths are prone for ABS problems you need to remove and clean all the sensors and then see if it's ok if not you need to trace the fault sensor, replace the coolant hose and fill it up with the correct mix of antifreeze. do a compression test and post the results, overheating has nothing to do with the boost the car may have been misfiring dur to headgasket faliure or any other reason but boost pressure is not the cause of your problems