sorry to be a nuisance again....
I had a waterpump leak on my opti which caused these kind of symptoms. It would not leak until there was pressure in the coolant system. It would then leak out of the weephole and get the opti wet. It would drive fine, but if I put the engine under load it would sputter and miss. Look carefully at the weephole of the opti, or start the car while it is on jackstands and see if you can see coolant where it isn't supposed to be.
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