Wireless on/off when security enabled

I have a Dell Dimension 8300 that has been beefed up to 768 MB RAM running on Win XP SP2 and I'm using a d-link airplus g dwl-g510 PCI card and a Dynex Wireless G Router. I hooked everything up, got it all configured and it works GREAT! Never a dropped signal or any disconnects. Had NO problems what-so-ever, until I decided to secure my network.

NOW... I take my old PC and connect it directly to the wireless router, so I can configure it for security. After running for a couple of weeks without it and having some issues with my PC (malware/spyware), which I erradicated using the processes outlined on this forum AND wth some Senior Support guy, I figured I'd better enable the security. I set up both my router AND my PC/card for:
Network Authentication: WPA/PSK
Data Encryption: AES
Then input my OWN "Network Key" which is a alpha/numeric/special char combo, so is pretty secure.

AGAIN.... everything works fine, I log onto the system and it's running great.... for a few minutes, then I lose the signal and have to click on the icon in my task tray and RECONNECT.

At first I thought I might be an issue with cable/internet signal strength, since there's only ONE cable input into the apartment and I've had to split it several times before it reached my back bedroom. So, I moved my cable modem and wireless router to the main cable input into the apartment and now it's actually doing worse and disconnecting more often.

A REAL PAIN IN THE RUMPUS! Would a firmware update on either wireless component help? I see other wireless networks within range of me, which means they must be in my apartment complex, but I think they maybe using a lesser security setting, since theirs are not identified as WAP, like mine is in the display.

Any thoughts? Suggestions? Help? Greatly appreciated...

JP