IP Address in browser address bar

I have recently migrated my websites from Apache2 to Lighttd due to severe memory problems.

However, depending on some factor that I am unsure about, users get the IP address of the website in the browser Address Bar rather than the FQDN. This occurs even if they entered the FQDN in the first place.

The set up has a number of Virtual websites (www.bigjimny.com, www.2499atc.org, www.sportsboatuk.org). In Apache I played with .htaccess on each website to overcome this, however Lighttpd only has a single conf file across all websites. Therefore I am at a loss as to how to do this.

1. Is there a specific cause of the problem in the first place, some browsers on some machines seem to get it correct

2. Is there a set of re-writes in the conf file I can use to overcome this.

I also seem to have issues serving pdf files, which again points to a rewrite problem?

Martin