I have error codes p0300, p0303, p0306. I have since learned that this is multiple/random cylinder misfire, cylinder 3 misfire and cylinder 6 misfire. Anyone know what the most common causes of this would be? The problem started as I was merging onto the freeway at full throttle, from about 30mph to 75mph. Recently cleaned the ETM with carb cleaner and brush, 300-400 miles before the problem occurred. I believe there was some residual fuel injector cleaner in the tank at the time. I'm thinking the most likely cause is bad spark plugs. Car has 114,xxx miles. Recently purchased and unsure of past maintenance. On its way to a new timing belt to make sure it stays alive.

 

i had that problem. it might have to to with the coils that are sparking them.
i had my coils start going out one by one around 130,000mi so after 2 or 3 of them i went ahead and had the rest of them replaced and it never happened again.