are you using roms or retail carts?
if your using roms have you updated your firmware for your flash kits?
i have seen mk fail to connect to my wireless when sitting 2 feet away from the access point. it seems that the game just has some bad wifi code built with a mind of its own. it is the only rom i have had issues with, but when it connects its all fine. i never disconnects, drops out, or anything like that.
if its a rom you might want to double check that its a clean rom and that you know where you got it from. i personally would NEVER run a rom on my nds that i got off some random site. infact i don't, never will, never have for many reasons. one being security and the other being that you don't know if it was hacked to perform in any way. the box of all of the slot1 carts always say "supports clean roms.'
with that said i can confirm that the mk track hack does perform perfectly on both local play and wifi play. i often select tracks i know others can't and imagine if they smile when they see it pop up on their screen for play!
-aw