'08 9-3 Turbo X odd misfire
 

This all started with my TX idling rough, and throwing a misfire code only at idle when the car was not warmed up. I replaced the Fuel Pressure Sensor, and the idle smoothed out. Power is normal and fuel economy is 21-23 MPG. All seemed well.

Flash forward about 500 miles, I started the car and it sounded more rough than before at idle. I drove a bit to see if it smoothed out at higher RPMs (as it did when I had the issue before), and now it violently shakes if I give it any gas, and it throws misfire codes again. I stopped the car and restarted the engine, all is back to normal. Smooth idle, no power issues. Another 500 miles or so of trouble free motoring, and it did this again, and again I restarted the engine and it's fine. Any ideas of what could be causing this would be a great help. It's stumping me since it isn't doing it regularly. The only other odd thing I noticed lately is that it makes a raspy noise at about 1,100RPM in gear (kind of sounds like an exhaust leak or something).

Thanks!