serious overheating 98 deville northstar engine
 

My 98 deville with the northstar engine is overheating badly. I have had the water pump, thermostat, radiator, all replaced. The mechanic can't find what's causing this, it has gotten as high as 250 before the messages come on. They also replaced a computer chip in the message system.
Anyone any help? Thanks, I am going broke!:eek: