Its either fuel or electrical issue. You can do a resistance check across the coil packs to determine if one is abnormal reading compared to the others as well as checking the coil pack and fuel injector connections as Grimm recommended. What makes this difficult is it is intermittent. It would be great if you could troubleshoot it when the issue occurs. It appears that it happens after you have been running the car for some amount of time so it could be a combination of heat and / or vibration. Good luch