Also the gasket theory, you mean there is unmetered air leaking thru, but this can be tested by spraying carburettor fluid around the gasket, if the RPMs go high when you are doing this, there is a leak in the intake gasket. Does the car idle fine in the first 5 minutes, and also the throttle response is normal ? Like you can get >5500 rpm without flooring the gas pedal ? If this is the case and you get high RPMs after 5 minutes, it is less likely to be the gaskets. And if I think more about it, if there is a leaking gasket, why should RPMs first go high and then die.

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Saumil S. Patel