egr valve

The best way to test whether or not your EGR valve is causing the codes is to open it with a vacuum pump and then to QUICKLY remove the vacuum line so the valve snaps shut. If you don no have a vacuum pump, rev the engine in park and then QUICKLY remove the EGR vacuum line so the valve snaps shut.
Then clear your codes. You can do this by disconnecting teh Negative battery terminal for 1 minute. Then drive your car with the EGR valve vacuum line DISCONNECTED and see how it runs. If it runs perfectly with no misfires and no hesitation, I would say that the EGR valve was a problem. If the misfire code comes back then look at a misfire article I wrote. Some folks find it helpful. Also check the EGR code P0401 for Toyota.

http://repairpal.com/what-does-the-term-misfire-mean
http://repairpal.com/OBD-Code-P0401-Toyota

let me know if you need more help Happy Holidays!

That is just one of many things that could cause a P0300 code.