DOS attack "slow post" : How to prevent in IIS

IIS doesn't have any rate throttling natively (or I guess it's negative rate throttling in this case). You can check out the Dynamic IP Restrictions module (http://www.iis.net/download/DynamicIPRestrictions). I don't believe it will check this specifically, but it's worth a peek.

Checks for this may stand a better chance on your firewall IDS filtering. There may be support there for checking this type of attack.