Mazda Repair: 1999 miata, best guess, coil pack

Question
About 4 months ago my Miata started having the following symtom. On a
very irregular time frame, the engine begins running very rough, the engine
check light comes on, and the engine almost stalls at idle unless given gas.
Often, after 1-5 minutes the problem is gone and engine runs fine for days at
a time. The problem has been occurring more frequently and lasting longer in
recent weeks. My local repair shop reports that the computer code, PO300
indicates a random misfire. Their best guess is that replacing  the coil pack/
igniter unit  might solve the problem. The symptom used to occur mostly
when the car was first started, recently it seems to happen when traveling at
any speed.
Thanks for any insight you can provide.

Answer
Several 99 and 2000 models experienced coil failures.  Before going worst case though I suggest installing a new set of NGK wires and NGK spark plugs if you haven't recently.  A breakdown of ignition wire shielding or a crack in a spark plug can cause similar low speed misfires.   Did they tell you which cylinder is misfiring?