Has the ECU been chipped? I had this problem and it turned out to be the ECU at fault. Someone had chipped previously, very badly and it caused all sorts of damage. Had to re-ring the pistons and glaze bust the bores.
Also do a test of the Air Flow Meter. There is a procedure in the Haynes on how to do it with a multimeter. If the AFM is at fault, the engine will never run right. Also check all vacuum, air and sense lines from the injector for air leaks.