Ok, basically, you bought a wireless router, set it up, it worked fine after initial set up, and next boot up its slower than molasses in January. And now you expect some guru here to consult a magical crystal ball, at a huge distance, and tell you exactly what is wrong.
Sadly, troubleshooting does not work that way. And you have a troubleshooting problem.
But it strikes me off the bat, you should be isolating the problem. So step one is to bridge around the wireless router and see what your internet connection will do when its directly connected to just one computer. That gives you a base line, maybe your connection coming in is really terrible, and no router on the planet can cure that. And if that is the case, you have to find the cause of the slow connection. If you do have a good connection coming in, then you can blame something in the router set up.
But I hope that gives you an idea of how trouble shooting works, you make a hypothesis and then test reality, and in that way, you narrow down where to look.
But on to step troubleshooting step two, and here I am assuming you tested your internet connection and found it good, its time to quit getting fancy with any special game port settings, just set up a default set up. And see what you get at various times of the day. Try different router channels. But there is a good chance there may be other wireless networks in range of your network, and your computers may be confused
on what network to join. So setting up some static IP's on your computer or wireless security could help if that is the case. Or other computers in the building could be joining your network with the same results.
And again, its a testable hypothesis that moves you forward in helping to tell what trees to or not bark up.
But what I have suggested is just a start in a long process. It would help if you described what you have in far more detail. And others more expert than I am may have a pile of other ideas.