RPM wont stay still. (Is this normal)

I was wondering about why my RPM wont stay at a certain spot when i rev it.... When i rev the car up to about 2K RPM and leave my foot there, the needle would go down ABOUT 600rpm's...... it would usually go down little by little..... Example... 300 rpm then 200 rpm then about 100 rpm, any clue to why this is doing that or if it's normal.......(i could hear the engine gradually hum down with the rpms)

Only mods are CAI, but it did this before i installed it also....