1992 240sx idle issues appear after time

[Dizel01]

This usually happens only when i been driving for an hour or so but when I stop and idle in neutral, my rpms drop to 500, making a light buzzing sound then bounces to 800. Then it falls back down to 500rpm, makes that slight buzzing sound then bounces up to 800 again. Its weird cause this doesn't happen when I first start the car and drive for a little bit, only when i drive for over and hour or so. I checked the vaccum lines....could it be the IACV being gunked up and needing a clean??

[180fan]

very plausible mafs issue. Give it a clean anyway.