I have done some reading up on stuff and I am more convinced that the issue is with the extra address you have got.
Basically, from the config you have got 188.8.131.52 255.255.255.252 on the external interface with 184.108.40.206 as the default gateway. The 255.255.255.252 subnet mask will only allow two possible host address which you are now using.
The additional address, cannot be added to your pix's external interface through the static statement because its not on the same subnet so there is no way that with IP Routing the traffic will ever reach your address.
You could do with speaking with the ISP to find out how you can resolve this in order to have two hosts address on the same subnet for you to get this working.
The only other way around it is to either setup on of the hosting sites (that you limit to the two IP Addresses) onto a different port and let the users know that its accessible via a different port to port 80.
If you have the resources then you can let the host headers be handled by something like ISA Server which can then redirect to different servers.
If any of this doesn't make any sense then please let me know.