Sounds like your AAC valve.
When I had a bung AFM in the GT-R it would rev to a certain point then cough/flutter and top out at say 2500rpm, load also caused it to do this. It idled fine though.
What you're describing sounds more like an AAC valve clogged up. Very easy to clean, do a search it's been covered to death. I've done it personally when I had similar issues and it worked a treat. My idle was stuffed, all over the shop. Very common.