[Aussie Mike]

Might be a dodgy MAF sensor. When you are just driving around normally the ECU uses the Mass Air Flow, RPM, TPS, MAP and O2 sensors to calculate your injector pulse width. This gives the best economy, emissions drivability etc. When you stick your boot down the ECU usually goes into speed density mode where it just runs off RPM, TPS and MAP and ignores the rest. At least that's how it does on an LS1 ECU.

Cheers