2005 RX8 Idle jumping between 500-1500 rpm, sometimes stalling
 

I have a 2005 RX8 auto, with a rebuilt motor that only has about 50 miles on it. The car itself has just under 125k. It's throwing a trouble code of P0300, a random misfire. I've already checked for vacuum leaks and most of the other common causes that it could be. I've already searched the forums here and didn't really find anything similar to mine. It starts just fine no matter what, but it idles really rough and tries to die after coming to a stop. The misfire pretty much clears up by the time it hits about 4500 rpm. However, when stopping after it's been driven hard, it's almost guaranteed to die. The only thing I can think of is if the idle air control is messing up, or maybe a bad O2 sensor, as the previous owner had a clogged cat and simply broke the catalyst out. I'm thinking that when the cat went bad, the O2 sensor did too, but it's not throwing a code. Another idea that I had was maybe the engine still needs to be broken in. I'm really not sure, and appreciate any help! thanks guys