j7 gkk

when i start the car up in a morning sometimes the car starts and the idle goes down to about 400rpm then slowly back upto 1200rpm, and sometimes when i open her up a bit then come to a standstill it drops down again, but it still pulls very well, ive checked the maf by unplugging it and the car dies so im presuming it isnt that, just wondering if anyone couls shed any light? im replacing the plugs and leads anyway, but just wondered if it could be anything else? cheers