i used to own a 3000gt that required premium gas only or it spark knocked and pinged like no other. after some research i learned that most sports cars (or “finely” tuned engines) require a higher octane due to engine design and tune. i figured it …. A leaking FPR does odd things. Gas in the vacuum line to the intake is cause to replace the FPR. A gauge on the feed line to the fuel rail eliminates several possibilities, but all seem very small chance with your symptoms. … …

Here is the original post:
very weird problem….long explanation – MX-5 Miata Forum