Thanks for all of your suggestions.
I should have mentioned that when I did the timing belt, I replaced the plugs, the water pump, the thermostat, the radiator hoses, the PCV valve, the oil, the oil filter and the air filter. I also flushed and replaced the coolant.
I guess I could have replaced the spark plug wires too, but I replaced them about 30,000 miles ago and the problem doesn't seem like a miss to me. Again, Miata was running fine before and after timing belt replacement. While the CEL came on, it really didn't start running crappy until the cleaning of the intake manifold, and the CEL has not come on since that manifold cleaning service. It runs badly only under certain load conditions, especially 30 mph in third gear, on a level road or a slight grade downhill. Under these load conditions, it feels as though someone is turning the key off and on. Depressing the gas pedal slightly seems to make it come back on, but don't be taking taking a sip from your coffee cup at that moment.
I bought fuel filters for both my 2002 Miata and this troublesome 97 but have not installed either yet. I must say that I'd be thinking the new fuel filter would solve the problem if it were bogging down under heave acceleration, but it seems to be having this cutting out or fuel starvation problem at relatively light engine loads (especially bad a 30 in 3rd).
I'm confident the spark plug wires are going to the right plugs.
I'm confident that I got the timing marks right when I installed the timing belt. I'm thinking it wouldn't be running well at any load if I had mucked that up.
I guess I'm going to bite the bullet and remove and clean out the intake manifold myself. But I might try replacing the spark plug wires and fuel filter first.