The problem resurfaced again today. I powered off the router for at least 30 minutes as I was testing my DrayTek router. When I powered the D-Link back on, LAN 1 which is connected to my PC's ethernet port was flashing intermittently. Where as LAN 4's light (which isn't connected to anything) was constantly on, no flashing light. So I connected it to my PC and it works again. I'm sure I tested all the LAN ports before when I had the 'flashing' light (rather than blinking, which to me shows an active connection).