So, this is when the engine is warm?

I assume that you are using uberdata for engine management, correct? In that case, the engine will idle in a strange manner where the idle bounces between 1500 and 2000 rpms when cold and then when warm, it will idle around 1100 when the car is put into neutral and then slowly go down to 750 rpms. If you notice this, it is completely normal.

If it doesn't do this, there may be a vacuum leak. Did you change the manifold gasket before you put it on?