Not sure if you've done it yet, but first thing you should do is to go to your local dealer and get your PCM reflashed. The newest program for your car addresses the P0300 random misfire problem. Spark plugs can also cause a random misfire, but unlikely if you've got low miles. Injectors are also a problem, especially if any additives have been added to your gas tank. Sticking injectors will cause the code to set at idle with a warm engine, and not at high speeds, as is your situation. This would be my guess as the cause of the code. If you can get your car scanned, PM me the freeze frame data and I can see if I can help you more. First, the PCM reflash.