Router seems to be limiting broadband problem

[m|ke]

Hi,

I have a strange problem with my router. I have 20mb virgin media broadband and its connected to a wireless Linksys router with ethernet cable directly to my pc.
The problem is that if I reset the router and modem and boot the net and do a speed test I will get full speed net @ 20mb , but if I then do another test straight after its @5mb also if I reboot after doing the first test it will be at 5mb. Strangly aswell if I change the ethernet port on the router to be plugged into say 3 instead of 1 the speed will go back to 20 then go to 5 as before.
This is after a clean format btw.

Plugging directly to modem gives 20mb, any ideas? This router used to work

thanks
Mike