Http 401 entries in IIS log of SharePoint Server

Question

I am trying to fix a performance problem on a WSS 3.0 clustered site.

Each server has ca. 30000 requests per hour, but ca. 90% are of type:

2010-10-13 10:00:05 W3SVC14309999999 servername 10.192.63.112 GET /sites/xxxx/_themes/Lacquer/viewheadergrad_lacquer.gif - 80 - 10.192.39.141 HTTP/1.1 Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.1;+HO32602R;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727) MSOWebPartPage_AnonymousAccessCookie=80;+WSS_KeepSessionAuthenticated=80 http://yyyy/sites/xxx/main.aspx ccccc 401 2 2148074254 291 521 15

Anyone know what could be generating these requests and why they are getting http 401.

Edit

We are trying to find out what authentication method is used, it is either NTLM or Kerberos in a Load Balanced environment.

Edit 2

There appears to be a pattern with 5 calls that return http 401.2, then 1 call that returns http 401.1, followed by a call that returns http 200.

Edit 3

We are using NTLM authentication

Answer 1

We fixed this by going over to kerberos authentication.