Can only visit HTTPS sites, not HTTP

For the past week or so I've only been able to access HTTPS sites in the evening between about 10pm to midnight. The rest of the time it seems to be fine.

At first I thought all internet was down, but then I realised it was just HTTP sites that weren't loading.

I've googled this and it seems like an known problem. The only difference is it only seems to happen in the evenings - about 10pm to midnight.

I've tried running spyware tools such as SUPERAnti Spyware and Spybot Search and Destory with nothing detected.

I'm using Windows 7 and have MS Security Essentials running.

I've tried connected via the WiFi and via an ethernet cable.

It seems to be affecting the other laptop in the house and the phones in the same way, although the other laptop is now broken so can't verify this.

Any ideas of what it could be?

Thanks.