DLink router issues

Within the last two weeks or so, my internet has been up and down like a roller-coaster. At times, I would be downloading at 500+KB/s, other times I can barely stay in any of my online games for more than a minute or two without dropping either from lag or a complete disconnect. Originally, I thought it was because of my roomate and his BitTorrent; he had 20+ downloads going, but the speed was only ~30KB/s download and ~10KB/s upload (though it may have simply been all of those open connections giving the wireless a hard time). My speedtests ranged from ~90KB/s to ~250KB/s with his BitComet running, and a solid ~250KB/s with all computers but mine off. Ive run several virus/spyware scans, and I dont have any P2P software running. When I connect my laptop directly to the modem, it's getting ~600KB/s speedtests. Unfortunately, I have no way to be sure whether this is a problem with my computer or the router since my laptop cant use wireless and I dont have an extra ethernet cord (to use it through the router), and my desktop is too far away from the modem to connect directly to it.

My DLink is a DI-634M and has the latest firmware. Is there something else I could be checking for to get my speeds to where they should be, or is there something in the combination of using the wireless itself, using BitComet, or having 4+ computers on the wireless connection that would be giving me these problems?