Hi, my IP address keeps changing.. to the wrong IP

Your ISP would not have the power to update your IP address information at DynDNS.
Nevertheless, if the IP address information for one of your hostnames keeps changing, there is still another Update Client in the game, most likely not from within your network, because the IP address is "wrong" from your perspective.

And the good news, you even know from what IP address your hostname has been updated. You can narrow down to whom this IP address belongs, using WHOIS and reverse DNS lookups and GeoIP research. You will find out at least the ISP and location of this IP address, which can give you an idea who or what is behind. Did you ever give a router or computer away, or left it when you moved?

And even if you're not able to find out what is updating your hostname, simply change your account's password at the DynDNS website and in your Update Client, and the phantom should have gone.