D-link DI-604 router problems

So i decided to have two computers at home and consequently two interent connections. I got this DI-604 router at Satrun and setup the wiring. Well it works, i have network connection on both computers, i can browse web on both, ... no problem. Problem shows up when i sign-in to webpages such as yahoo, ebay,... When i punch my password and press 'log-in', the interent explorer(7) starts to working over and over but nothing happens and finally the famous 'page not found' message appears. Even in Toytown i can not post or send any PM ! The problem disappears when i do not use the box and connect the main network line to my computer so i guess the box is blocking some traffic. Looking into settings of the router the firewall is disabled.

I even got another router with the same model number yesterday and tried it. Same story.

I appreciate if anybody with the same experience shares his/her expertise with me.

Thats weird. It might be to do with MTU size, I've had problems using dlink stuff before, and it was always down to the MTU.
Have a look here http://www.dslreports.com/faq/tweaks/4._MTU to get a walk through on changing MTU size. It might well do the trick. Its all a bit techy I'm afraid.