engine is bouncing off idle when revved
 

When in neutral, if I rev it a little, the engine will slowly bounce off idle up to 1400, then to 1000 RPM. It will continue to do so for about 10-15 seconds, then finally drop down to normal idle speed. I'm not getting an SES light, and the car seems to run strong, though. Mods are in my sig. Does anyone know what may be the cause to this? Thanks!