A high idle like that upon startup is normal. Since the motor is still cold from sitting, your IACV opens up the butterfly valve slightly to help the car warm up. Once it's warmed up, the valve goes back to its minimum position and you get a normal idle.
However, a fluctuating idle can be a few things, but 95% of the time it is a dirty IACV. I would clean that first, and see if it works.
Here's a reference
Cleaning your IACV *Great Write-up*