So...new at this. I hope I am posting this properly.

My uncle gave me his 99 Miata. It sat a year in his garage and would not start when I tried it. Dead battery replaced.... cranked/ no start. Pulled plugs and no spark on 3 and and intermittant on 4th

Wires and plugs replaced ...started and ran smoothly for about 20 minutes then died......and now is back to no start condition.

Ideas where to look?

 

You need 3 things to run an engine: fuel, spark and compression. And they need to come at the correct time.
Spark is good. but since you replaced the plugs and wires. I would also replace the coil packs to make it 100%.
Compression seems good. I take the fact that it ran smoothly for 20 minutes as proof positive.
Fuel could be the problem. The gas was old and stuff may have settled in the tank. After running a while the pump picked up stuff and clogged the filter and/ or the injectors.
Replace the filter, dump the old gas and get some fresh fuel and add a fuel injection cleaner (gum out, maybe). Then cross your fingers and crank it.

 

welcome. there are many peoplewith special knowledge, they will give you the profesional advice