Active Directory problems: Some PCs take 15 minutes to logon in the morning. Any advice for figuring out what's causing the problem?

I have an environment with about 50 PCs. There are 2 domain controllers (Windows 2008 R2).

A few weeks ago, I created a 3rd domain controller. I let the environment run for a few days, and then ran dcpromo to demote the initial domain controller

So originally AD01, AD02. Now, AD02, AD03.

The reason was that AD01 was setup as a Windows 2008 Server Core and I needed to have a full Windows 2008 server, so AD03 is a normal Windows 2008 server.

Since I demoted and removed AD01, I keep getting weird problems. Every few days, the PCs will take up to 15 minutes to logon in the morning. This is a mix of Windows XP and Windows 7 Pro.

I've gone through everything I could think of, but can't figure out what the problem is.

There were a couple of PCs that couldn't logon at all, or when they logged on, none of the users desktop icons or My Documents folders were there. We are using folder redirection via GPO.

I found that those problem PCs had hard coded DNS server, which included AD01. Logging into local administrator and setting DNS server to be acquired via DHCP fixed the problem for those couple of PCs.

The weird thing about this is that it does not happen every day. It only happens every 2 or 3 weeks.

I've looked at my Cacti graphs of the domain controllers and don't see anything out of the ordinary in terms of processes, CPU, memory, or bandwidth.

Also, is there a way to make sure that IP Address, and DNS server are set to DHCP with a GPO?