Random misfires are not typical symptoms of a vacuum leak or MAF issue. Bad plug or coil would be my first thought, but the plugs are new and, if it was a coil, I would think (1) you would be getting a misfire code for a particular cylinder, but not random misfires, or (2) you would see even more misfires while under heavy load.
The ECU doesn't sound too likely either because it runs OK under load.
I would check fuel pressure. JTIS claims P300 may be thrown for fuel pressure that is too low or too high.