Are you using the built in wifi to connect or the lan adapter? I used to get a lot of such errors with my wifi, I read somewhere Bout changing the wireless channel, I did, never occured again since. I changed it to channel 11.

 

Yes, it's a router setting. Something your neighbors are doing could be using that channel now (that they weren't before), causing some issues.

In your start/globe menu, go to the "run" command. If you're on vista/7, you'd use the little white box near the bottom. Type in "CMD" and press CTRL+SHIFT+ENTER.

In the black box that comes up, type "ipconfig" and press enter. It'll list a series of four numbers as your "default gateway". Write those down. Go open any web browser, and enter the four numbers (like 192.168.x.x) into the address bar, and press enter.

A login box should come up. The username should be "admin" or blank, and the password can be anything from "admin" to "administrator" "pass" to "password" or "password1", or something else if you've set it differently. Ask whoever set up the router what the password to get into it is, or check this page.
http://www.phenoelit-us.org/dpl/dpl.html

The find the wireless channel and change it a few numbers up or down.

EDIT: Right now my DSi's telling me the shop is under maintenance.

 

Are you afraid nintendo auto update with new console 4.3?