Intermittant Power Loss

Just purchased a 2000 Miata with 80K miles. All 60K maintenance performed at Mazda dealer for big $'s by previous owner. Runs great 99% of time. Ah, the other 1%, it feels like running on half power. My cheap code reader says multiple cylinder misfires when problem occurs. Shows up when I start warm or hot, never after sits overnight. Clears out after a mile or so and runs sweet. Cleaned the EGR valve, didn't help. Used a can of injector cleaner, didn't solve. Any other ideas? Thanks