The client PC's on our small business network are experiencing very slow internet resolution speeds on a wide range of web addresses. The clients are Windows 7 & XP running on new and older PC's and laptops. To give a background on our network, I have an ISP 6up & 6down connection into our Fortinet Fortigate 60c Firewall\Router which in turn connects to a Netgear JGS524e switch. I have SBS 2011 Standard running on a Dell T710 server and it is connected to the switch. DHCP is allocated to the 6 client machines via SBS server using MAC address reservations. (This is just done out of habit at this small business.) The firmware on the router and switch are up to date. Each client machine uses SBS server for DNS and I have our ISP's DNS server setup as forwarders in DNS manager. I also flush the DNS cache on the server on a daily basis and randomly on the client.
The problem we are experiencing is internet sites can take 10-60 seconds to load and this occurs on a daily basis. I have spoken to the ISP and they sent out engineers who checked the connection and said everything is fine. When I do speed test at www.speedtest.net
I can get results anywhere from 0.5 down & up - 6 down & 6 up however this changes all the time and can greatly change from machine to machine and hour to hour.
I this stage I need guidance as to what else could be causing this problem.