Ok..I'm assuming you both have other threads here? Plaese link them because most of us don't want to tell you to do the same stuff we might have already suggested. It'll just aggrevate everyone. Please link em if ya got them.
Ok.. Misfires eh.. P0300 hmmm
Keeping in mind this is for a rear wheel drive GTO, use the diagnostic aids to think of things in our cars that might be similar.
GearChatter.com :: View topic - DTC P0300
Like bad motor mounts etc etc...
Then look at your O2 wiring, make sure it's not touching any plug wires. Sounds like you've both gone through the usual suspects of tune up and vacuum. Check the elbow at the brake booster, it's a check valve. When bad it's a vacuum leak. Being P0300 it's random multilpe which usually means it's something else goofy.
Did it start happening after a battery cable unhook? Try and case learn, it's a 1 in a 100 chance, but when the pcm loses it's position..it does all sorts of f'd up stuff.
