Originally Posted by Cyber370

P0300 means you have a random misfire not related to just one cylinder. If it was just one cylinder it would give you P0301 for cylinder one, P0302 for cylinder two and so on. So I need to ask, did you get a blinking check engine light while driving? If you did, do not drive the car. You may be causing damage to the cats and more if you continue driving it.

I had to learn the hard way on how to track down the cause of a P0300 code on my previous LS1 Vette. After changing everything there was to change, turned out to be valvetrain related (bad valve springs) and I traded it in at that point.

Like others have said, do not just throw parts at it. You may waste a lot of time and money when the culprit can be really simple.
Start with taking care of the new P1223 code. This has to do with your throttle position sensor. As suggested by others, make sure your connections are good. Once you're sure, I would try going back the stock intakes and clearing the codes. If they do not come back, you found your problem source.

When codes pop up, you always start with investigating recent changes and mods.