I don't use wireless so I'm mainly guessing here. I think that at 30 secound intervals your wireless card is pinging (checking it's status) your wireless router, this is done whether you are using the net or not.
It sounds like this is stressing your system, which causes the glitches for those few secounds.
There may be a symbol in the notification area of the taskbar, which will enable you to temp disable the wless card.
Hope this helps (course I maybe completely wrong, but gets you started).