Intermittent problem when browsing the internet, it's pot luck if a website loads or fails

We have a client with a Windows Server 2012 R2 with a number of PC’s & Laptops with a mixture of Windows 8, 8 and 10.

All users are experiencing an intermittent problem when browsing the internet. Some sites, even www.google.co.uk sometimes loads and other times we receive an error. If we reload / refresh the website, sometimes it will now load.

We experience this problem with IE11, Google Chrome or Firefox.

When a site loads in one browser, it often won’t load in one of the other browsers.

Having used chrome://net-internals/#dns in Google Chrome this shows that the DNS is not being resolved, sometimes, when the page is reloaded, the DNS resolves the correct IP Address.

Often, when we can access www.google.co.uk, if we perform a search, the search result does not display due to the same error.

Some PC’s have static IP Addresses and others are using DHCP.

The internal IP Address is using 172.16.20.xxx (this was set up by the previous IT Company) and we have the gateway set to 172.16.20.1 and the server is on 172.16.20.20. There is also a telephone system on the network. We have recently replaced the ADSL Router to try to resolve the issue.

Some of the PC’s with static IP Addresses have the following DNS Servers registered :-
172.16.20.1      (Router)
172.16.20.20      (Windows Server)
62.6.40.178      (BT Primary DNS Server)
62.6.40.162      (BT Secondary DNS Server)
8.8.8.8            (Google DNS Server)
8.8.4.4            (Google DNS Server)

BT & Google DNS Servers were added to try to resolve the problems, but we are still experiencing this intermittent problem.

Has anyone had a similar problem ?