Your problem is ignition related. The 99/00 coil design is **** and its a known problem. Mazda refused to acknoledge the problem and issue a recall but instead they quietly revised the coilpack and changed the PN. If you are still running the factory coil after 14/15 years, its time for a new set of coilpacks.

The failure mode looks like this:

1. Coil pack gets flakey, causes missfire
2. Missfire makes car idle rough, dumps unburned fuel into exhaust under load
3. Surpluss of unburned fuel makes cat go chernobyl
4. P0421

IGNITION COIL (BP4W-18-10XB-9U) - $305.43 - BP4W1810XB9U

You are going to need to replace your plugs, wires and front o2 sensor but this will not solve anything until you replace the coilpack. Its time to spend some cash and refresh the 14/15 year old ignition components on your car, they don't last forever.

Part of me agrees with 18 because this information is readily avalible if you search, not only here but miata.net, miataforumz.com and clubroadster.net. If the clubroadster guys can figure this out, you can figure this out.

I'm throwing you a bone because I'm in a good mood, and I hate that ******* precat.