cannot view some web page and ftp download

Hi,

this is our simple environmet :

Remote Host<>Router Imola with Vpn SSL <>modem adsl< Internet >modem adsl <>fortigate c40<>switch 10/100<>Local host

this is our problem:

- We cannot view some web page and sublink

- We cannot download data with ftp or other protocol.

the problem seems repoted to tcp-mss misconfigured or mtu, we try to change this value on interface or firewall policy without success. If we isolate the firewall the problem does not occur.