Poor accelaration at about 1800 rpm , flatspots and pinking

[LADDOG]

1994 M 2.0l turbo auto Wentworth.

On accelaration I have flatspots and poor accelaration at about 1800 revs if you put your foot down it can normally be driven past this point and it runs smoothly until the revs drop back down to 1800. It also pinks at higher revs with metallic sound and the plugs show to be running on a weaker mixture.

The fuel/emissions warning triangle light is on and I have checked the onboard diagnostics which gives a code of 1-2-1. This points to the mass air flow meter which I have changed for a brand new aftermarket replacement. However, it still has the problem and the same code has been given.

I have also changed the fuel pump relay; still no improvement.

When the problem first showed itself the warning light was not illuminated it was only the next day that this came on.

As a bit of background, my wife took the car some 70 miles to Nottingham and on the way back needed fuel and put in super unleaded(97 ron) as that was the only fuel available in the garage. The next day i ran the car to London and it was only a few miles into the journey on the way back that the problem started. Would the different fuel have any bearing on this or where do I go next?