you should be running premium in any turbo motor.
regardless, make sure you're not off by a tooth on the distributor install, and I hope you used OEM Sumitomo wires.
every aftermarket wire Iv'e ever tried just doesn't seem to work as well.

I had an 83 Jeep CJ that sat for 10 months before putting it back on the road becuase I put a worked over EFI 4.0L in it . it sounded like someone emptied out a roll of pennies into the cylinders all the way to the gas station. then I put premium in it and all my problems went away.