This 3A90 Mirage without AS&G has no EGR system, only those with AS&G do have EGR.
I assume the valve-overlap creats enough EGR for the car to pass initial European type approval. That might explain the problem, because unfinished-burnt exhaust gasses blowing onto the intake valves will foul them up.
That local company with a small fleet of 3A90 Mirage's (called Space Star here) has gotten rid of their entire fleet.
I was told each one of their 3A90 Mirage repeatedly suffered the same issue. They now have a fleet of Suzuki Celerio.