Boxster Revs Drop

[PrestonV]

1999 2.5L Boxster
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Been having an issue with my Boxster where, on cold start, my revs drop as I lightly push on the throttle and there tends to be a knocking sound if I continue to hold the throttle down lightly when the revs drop. Usually the issue resolves itself after waiting a little and turning the car off and on but I'd like to get to the root of the problem. If anyone has any suggestions or tips as to why this is happening and a possible resolution I would be happy to hear them/