Irridiums are known to cause misfires on some GT4's, although others seem ok. I have also read somewhere that the GT4 requires a 'non standard' gap which is a different part No. If you try re-gapping you will damage the plug.
Idle speed is something the the ECU learns after a reset. Normally it errs on the high side after a reset, but I have heard of cars tending to stall until it's gone through the learning proceedure. It may be worth checking the adjustment of the throttle position sensor, and if that is correct adjust the throttle stop up a little the re-adjust the sensor accordingly.
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