I don't know if we have a thread dedicated to check engine lights somewhere here, I haven't found any, but if there is one, redirect me there please.

I have a 90 miata, 1.6l, close to 200 000KM, standard, all stock engine. Last summer I noticed that when I keep the throttle stable for a while (2-5min), mainly on highways, but also anywhere else if the throttle is stable long enough, the engine light goes on. I also noticed that as soon as I gave it throttle back, made the engine work a bit, the light disappeared, only to come back if I was going at a stable pace again.

My mechanic seems too busy to be bothered, or not smart enough to figure it out, and engines really, but really aren't my area of expertise. However, I assume it's got to be something of the likes of bad air/gas ratio when not straining the engine, or a bad air problem of sorts? Everything else seems to check out, my air filter is brand new, the air intake was cleaned last summer, spark plugs are doing fine, I mean the engine runs like a beauty, so I'm curious as to what that could be. I doubt it's going to make my engine blow up out of the blue, but if it's putting unnecessary wear on it, it would be awesome to know what it is, what to do. Anyone have similar experiences or know why it would do that?

Thanks