I've had the same symptoms: sometimes, when coming from higher rpms, it idles at 3000 for a while and then settles at 1500, or goes to 1500 after a few blips of the throttle.
I asked this question on the old NSR mailing list, and 3-4 people said they had the same symptoms and a few others said it is normal for a NSR.

Someone suggested that the flat sliders in the carb, could be sucked against the groove and bind a little.

I never did anything to cure it and don't even know if it still exists but taking the cases apart because of this, sounds a little over the top.

You can test the two outer crank seals by spraying some diesel start over them while the bike is running. If the rpms change, every time you spray, then the seals are bad, if nothing happens, they're ok.
This can also be done around the manifold or along the case joints.
Use a thin plastictube, so you concentrate spray where you want it.

Oh and ehhh, its highly flammable
_________________
Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom