Did you check the water in the radiator? Was there antifreeze in? Even if there is one won't know if it is the correct ratio.
I suggest get bottles of antifreeze ( my 4.2 takes 14 odd liters of water, so I bought 6 bottles Silverton Radiator recommended antifreeze ). Antifreeze is also an
anti boil as well as an anti freeze. Check how many litres your car takes, and try that before stripping the motor.
I would also flush the radiator before doing the above antifreeze change. It might be that the gauge isn't seeing it as a high temp, but the water is just at that point where it is boiling. Check hose connections. There may be a minute leak that sucks air in when the radiator cools down. This will cause the radiator to boil quicker.
As Peter says.... if there is no exhaust gas in the water.... the gasket can't be leaking.
