Exactly same problem with a Dlink AirplusG+ DWL-G650+ wifi pccard involving an ACX111 chipset.
Using Xubuntu 8.10 Intrepid with kernel: 2.6.27-9-generic #1 SMP Thu Nov 20 21:57:00 UTC 2008 i686 GNU/Linux
The firmware installed by default in /lib/firmware/acx/default is not the same one than
in previous Ubuntu version 8.04.
I recopy the old firmware to get exactly the same one than in 8.04 and reboot. Now the laptop
don't hang anymore after some minutes. The board is still activated, but the network does not work more.