Intermittant start...

[cam6063]

Hi,
Looking for some laymans advice, I have a 92 MX5 Eunos 1.6, 100,000 miles with intermittant starting probs.
Can be cold, warm, hot, any throttle position, new plugs and leads, new ECT sensor to resolve overfueling, engine when running is sweet, good fuel econ and performance, no exhaust plumes, good idle.
No other diagnostic code's in ECU.
Engine will turn but not catch but on other occasions it will start up first time. Belts done 18 months ago. I bought car 6 months ago and the problem seems to be getting worse. Any thoughts greatly appreciated! Frustrating!!!!!