D-Link DSL-604+ Question

I have a DSL-604+ and I had a small question, when I have a code set so not any can just access the network it seems to work just fine, however on my upstairs computer it will dissconnect and reconnect every few minutes and its starting to tick me off.

When I turn off the code protection, it seems to work 100% fine, its just we have idoits living beside us and they have no idea what paying for your internet means, same with someother people who live around the area. Well if anyone knows why please let me know.

Only thing I can think of is, I'm using a M$ routor for upstairs... Maby not fully com. with d-link.