You probably need to try a different channel.
What I tend to do is fire off an ongoing ping from the computer to the router, and watch for dropped packets and long return times.
From a cmd prompt in Windows, do:
ping -t [ip address of router]
and leave that going. You shouldn't see any pauses (which are dropped packets or delays), it should be a constant "reply from" every second, with responses times of no more than a few milliseconds (I am seeing 1ms here).
If there are delays, dropped packets or hundreds of milliseconds, then you should try changing the channel from the router.
You can leave the ping running. When you change the channel on the router, the connection will be dropped momentarily, but it will reconnect and the ping will carry on.
Most routers use channel 11 as default. If there is a problem on 11 I tend to try 1, then 6, then any others after that if still no improvement.