Trust me, the trouble is the catalytic converter. I had all the classic symptoms as everyone else and in the end it was the cat. P0300 is the cat. It causes backpressure to the extent the engine can not fire properly. That is why it does not show up as a particular cylinder misfiring. The engine is firing but not getting full power because it can not exhaust the previous cylinder fuel burn. It is not the secondary air pump, nor the 02 sensor, as I replaced all and still had the problem. Engine would not produce over 2500 rpm in driving condition. Felt lilke transmission was slipping. That is because transmission could not shift because motor was not developing enough power to get to the next higher speed.
The GM special policy number on the catalytic converter is 05551, which provides warranty on cat to 10 years or 120,000 miles. Good luck.