Here are some similar issues and fixes..
P0300 P0302 P0305 my experience and finally fixed
http://www.ttora.com/forum/showthread.php?t=175997
P0300 P0306 Misfire fixed - fuel injector '99 4Runner
http://www.yotatech.com/f2/p0300-code-bad-gas-165029/
Some other threads I found advised to: "look at the plug wires they are known to bust when old near the boot"
"When the code P0300 is set in the Powertrain Computer, it means that the Misfire Monitor has detected more than a 2 percent variance in RPM between the firing of any two (or more) cylinders in the firing order. The Misfire Monitor constantly checks the rotational speed of the Crankshaft by counting the pulses of the Crankshaft Sensor. The Monitor wants to see a smooth increase or decrease in engine RPM.
If there are jerky and sudden changes in the speed output of the Crankshaft Sensor, the Misfire Monitor begins to count the RPM increase (or lack thereof) contributed by each cylinder. If it varies beyond 2 percent, the Monitor will set a P0300 code and illuminate the Check Engine Light. If there is more than a 10 percent variance, the Check Engine Light will blink or pulse in a steady manner to indicate that a harmful Catalytic Converter misfire is occurring. "