directory services group query changing randomly

Question

I am receiving an unusual behaviour in my asp.net application. I have code that uses Directory Services to find the AD groups for a given, authenticated user. The code goes something like ...

string username = "user";
string domain = "LDAP://DC=domain,DC=com";
DirectorySearcher search = new DirectorySearcher(domain);
search.Filter = "(SAMAccountName=" + username + ")";

And then I query and get the list of groups for the given user. The problem is that the code was receiving the list of groups as a list of strings. With our latest release of the software, we are starting to receive the list of groups as a byte[].

The system will return string, suddenly return byte[] and then with a reboot it returns string again.

Anyone have any ideas?

(marc_s) Added code sample:

DirectoryEntry dirEntry = new DirectoryEntry("LDAP://" + ldapSearchBase); 
DirectorySearcher userSearcher = new DirectorySearcher(dirEntry) 
  { SearchScope = SearchScope.Subtree, 
    CacheResults = false, 
    Filter = ("(" + txtLdapSearchNameFilter.Text + "=" + userName + ")")
  }; 

userResult = userSearcher.FindOne(); 
ResultPropertyValueCollection valCol = userResult.Properties["memberOf"]; 

foreach (object val in valCol) 
{ 
    if (val is string) 
    {
        distName = val.ToString();
    } 
    else 
    { 
        distName = enc.GetString((Byte[])val); 
    }
}