PCI Wi-FI card vs. VoIP = ping lost

Hello !

Dear All, help is needed:

I have D-Link access point (DWL-3200AP) and clients with D-Link (DWL-6520) and Linksys (WMP300N) PCI Wi-Fi cards installed on PC's (WinXP Pro SP2).

The problem:
when W-Fi users is trying to use IP soft-phones (SJphone,X-Lite) the networking via Wi-Fi interface seems to be "frozen" - the link is up, but even pings cannot go. Any other software runs well.
The must interesting thing is that the problem does not exist when using USB attached Wi-Fi adataptors (but I must use PCI).

Any suggestions ?

Thanks

Denis