Sounds about normal, when cold, my VQ35 shoots up to nearly 2,000rpm, quickly drops back to about 1,500rpm, then slowly drops down to 650rpm as it warms up. if it is already warm, it probably only peaks up to about 1,200rpm at start and drops back to 650 within about 5 seconds.
I'm surprised a battery disconnect/reconnect helped, ECUs these days don't normally get amnesia that easily..