Miata engine missing

car idles fine at traffic lights, but upon startup it misses until it gets up in RPM

Year and mileage please.
Is this still under warranty?
Has all the maintenance been kept up to date?
What’s been checked so far?

I would start with new plugs and wires.  The Miata uses a system that fires each plug twice as often as most cars.  It needs new plugs and wires twice as often.  Use OEM brand plugs and wires.  They are the best in the long run. 

Having said that, don't skip the same mountainbike's response.

The logic of those “wasted spark” ignition systems has always eluded me. I just don’t see any advantage.

has 144K miles, all maintenance has been kept up, replaced plugs