I've had the exact same thing for a few weeks now in my 328. It seems to happen usually when coasting then then putting the clutch in around 1500 rpm, and if I keep the clutch engaged down to 1000 rpm as I coast it doesn't drop below 650 when I finally put the clutch in. I don't want to have to think about this every time I come to a stop though. No codes on the dash, and I've been thinking about getting a BT tool and doing some diagnostics but I haven't had the time yet.
I recently discovered that the previous owner did the intake "silencer" delete and there was a leak in the epoxy, so for now I siliconed the hole and I have a stock tube coming in in a few days. I thought that was going to fix it because it's after the MAF, and although it hasn't stalled since then it has dropped to about 400-500 rpm a few times. I'll see in a few days if the stock intake tube helps, but I doubt it.
Also just did spark plugs, filters, oil (I'm at 80k) and techron a few tanks ago. No noticeable change. I found a few other threads here with similar problems, but not any real solutions to this exact problem.
Any other ideas?