With some exact data (Router, Firmware, exact type of network and encryption method etc) it's easier to track down such stuff. :-)
Also, try disabling "Power Saving Mode" completely in Settings > Internet Connections for that connection. It may drain your battery faster, but it may make the WiFi connection more stable.
If you are tech-savvy, an ethereal/wireshark network dump is welcome too in bugs.maemo.org. :-)