Does this vehicle have an EGR Valve? It may be hung open. Check the Emissions Label under the hood of the car. The label should show the location of the EGR valve and any vacuum and or wiring attached to it. Get back to us.
http://engine-codes.com/p0300_toyota.html You will have some work ahead of you trying to figure this out. You may want to invest in a spark tester light. It clips onto the wires and lights up every time the plug fires. They make a similar light to test your injectors also. I would put in new plugs then test the wires. A bad Crankshaft Position sensor will also cause a misfire, which I didn't see mentioned in the above link. But there should have been a code for a bad CPS.
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