I'm having a similar problem in Feisty with the Madwifi-ng driver on a TrendNet TEW-443PI
Already ran memtest - no problems there. Here's what the kern.log outputs before I have to reboot:

Kern.log:
Apr 13 22:09:40 Sparta kernel: [ 8413.883195] BUG: soft lockup detected on CPU#1!
Apr 13 22:09:40 Sparta kernel: [ 8413.883223]  [softlockup_tick+156/240] softlockup_tick+0x9c/0xf0
Apr 13 22:09:40 Sparta kernel: [ 8413.883241]  [update_process_times+51/128] update_process_times+0x33/0x80
Apr 13 22:09:40 Sparta kernel: [ 8413.883250]  [smp_apic_timer_interrupt+112/128] smp_apic_timer_interrupt+0x70/0x80
Apr 13 22:09:40 Sparta kernel: [ 8413.883258]  [apic_timer_interrupt+40/48] apic_timer_interrupt+0x28/0x30
Apr 13 22:09:40 Sparta kernel: [ 8413.883271]  [_spin_lock_irqsave+47/80] _spin_lock_irqsave+0x2f/0x50
Apr 13 22:09:40 Sparta kernel: [ 8413.883280]  [<f89e28f4>] ieee80211_free_node+0x24/0x80 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883308]  [<f89dd6d7>] ieee80211_input_all+0x57/0x90 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883333]  [<f8b21fe1>] ath_rx_tasklet+0x751/0x7e0 [ath_pci]
Apr 13 22:09:40 Sparta kernel: [ 8413.883353]  [tasklet_action+99/224] tasklet_action+0x63/0xe0
Apr 13 22:09:40 Sparta kernel: [ 8413.883362]  [__do_softirq+130/256] __do_softirq+0x82/0x100
Apr 13 22:09:40 Sparta kernel: [ 8413.883372]  [do_softirq+85/96] do_softirq+0x55/0x60
Apr 13 22:09:40 Sparta kernel: [ 8413.883379]  [do_IRQ+69/128] do_IRQ+0x45/0x80
Apr 13 22:09:40 Sparta kernel: [ 8413.883387]  [common_interrupt+35/48] common_interrupt+0x23/0x30
Apr 13 22:09:40 Sparta kernel: [ 8413.883399]  [<f8a2bff0>] zz033ebfbf+0xc0/0x148 [ath_hal]
Apr 13 22:09:40 Sparta kernel: [ 8413.883414]  [<f8a3f3b8>] zz0b709eff+0x38/0x48 [ath_hal]
Apr 13 22:09:40 Sparta kernel: [ 8413.883430]  [<f8b16b77>] ath_txq_update+0x87/0xe0 [ath_pci]
Apr 13 22:09:40 Sparta kernel: [ 8413.883449]  [<f8a2ba46>] zz0067d221+0x1a/0x34 [ath_hal]
Apr 13 22:09:40 Sparta kernel: [ 8413.883463]  [<f8b16bef>] ath_wme_update+0x1f/0x90 [ath_pci]
Apr 13 22:09:40 Sparta kernel: [ 8413.883474]  [<f89e9ac3>] ieee80211_wme_updateparams_locked+0x123/0x210 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883499]  [<f89ea141>] ieee80211_wme_initparams_locked+0x41/0x170 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883524]  [<f89eb521>] ieee80211_wme_initparams+0x21/0x40 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883546]  [<f89e3a00>] ieee80211_sta_join1+0xa0/0x200 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883567]  [<f89ee1d0>] mlmelookup+0x0/0x60 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883589]  [<f89ee1d0>] mlmelookup+0x0/0x60 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883614]  [<f89eee2f>] ieee80211_ioctl_setmlme+0xbf/0x250 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883641]  [wireless_process_ioctl+701/992] wireless_process_ioctl+0x2bd/0x3e0
Apr 13 22:09:40 Sparta kernel: [ 8413.883650]  [<f89eed70>] ieee80211_ioctl_setmlme+0x0/0x250 [wlan]
Apr 13 22:09:40 Sparta kernel: [ 8413.883673]  [sock_ioctl+0/528] sock_ioctl+0x0/0x210
Apr 13 22:09:40 Sparta kernel: [ 8413.883681]  [dev_ioctl+531/896] dev_ioctl+0x213/0x380
Apr 13 22:09:40 Sparta kernel: [ 8413.883697]  [sock_ioctl+0/528] sock_ioctl+0x0/0x210
Apr 13 22:09:40 Sparta kernel: [ 8413.883704]  [do_ioctl+43/144] do_ioctl+0x2b/0x90
Apr 13 22:09:40 Sparta kernel: [ 8413.883712]  [vfs_ioctl+92/672] vfs_ioctl+0x5c/0x2a0
Apr 13 22:09:40 Sparta kernel: [ 8413.883719]  [sys_ioctl+114/144] sys_ioctl+0x72/0x90
Apr 13 22:09:40 Sparta kernel: [ 8413.883725]  [sysenter_past_esp+105/169] sysenter_past_esp+0x69/0xa9
Apr 13 22:09:40 Sparta kernel: [ 8413.883741]  =======================
Apr 13 22:35:43 Sparta kernel: Inspecting /boot/System.map-2.6.20-16-generic

Any ideas?
Thanks, 
-Matt