Closer to the source of the problem.
I deactivated the internal WLAN card of my Thinkpad T60 in BIOS configuration and used a external USB-Card for about 3 weeks without any freeze. While I was working on some virtual machines copied from external hard disks it froze again. 

I checked the kern.log and found:


Jan  2 11:53:50 ubuntu kernel: [ 1443.498163] Initializing USB Mass Storage driver...
Jan  2 11:53:50 ubuntu kernel: [ 1443.498532] scsi6 : usb-storage 1-5:1.0
Jan  2 11:53:50 ubuntu kernel: [ 1443.498731] usbcore: registered new interface driver usb-storage
Jan  2 11:53:50 ubuntu kernel: [ 1443.498734] USB Mass Storage support registered.
Jan  2 11:53:51 ubuntu kernel: [ 1444.501178] scsi 6:0:0:0: Direct-Access     WDC WD50 00AAKS-00YGA0         PQ: 0 ANSI: 2
Jan  2 11:53:51 ubuntu kernel: [ 1444.502002] sd 6:0:0:0: Attached scsi generic sg2 type 0
Jan  2 11:53:51 ubuntu kernel: [ 1444.506387] sd 6:0:0:0: [sdb] 976773168 512-byte logical blocks: (500 GB/465 GiB)
Jan  2 11:53:51 ubuntu kernel: [ 1444.509534] sd 6:0:0:0: [sdb] Write Protect is off
Jan  2 11:53:51 ubuntu kernel: [ 1444.509539] sd 6:0:0:0: [sdb] Mode Sense: 38 00 00 00
Jan  2 11:53:51 ubuntu kernel: [ 1444.509543] sd 6:0:0:0: [sdb] Assuming drive cache: write through
Jan  2 11:53:51 ubuntu kernel: [ 1444.512893] sd 6:0:0:0: [sdb] Assuming drive cache: write through
Jan  2 11:53:51 ubuntu kernel: [ 1444.512904]  sdb: sdb1
Jan  2 11:53:51 ubuntu kernel: [ 1444.526780] sd 6:0:0:0: [sdb] Assuming drive cache: write through
Jan  2 11:53:51 ubuntu kernel: [ 1444.526789] sd 6:0:0:0: [sdb] Attached SCSI disk

Jan  2 13:06:19 ubuntu kernel: [ 5793.020774] WARNING! power/level is deprecated; use power/control instead
Jan  2 13:06:20 ubuntu kernel: [ 5793.107742] usb 1-5: USB disconnect, address 6
Jan  2 13:06:45 ubuntu kernel: [ 5818.089093] usb 1-5: new high speed USB device using ehci_hcd and address 7
Jan  2 13:06:46 ubuntu kernel: [ 5819.067943] scsi7 : usb-storage 1-5:1.0
Jan  2 13:06:46 ubuntu kernel: [ 5819.068341] usbcore: registered new interface driver ums-cypress
Jan  2 13:06:47 ubuntu kernel: [ 5820.068602] scsi 7:0:0:0: Direct-Access     WDC WD16 00BEVE-11UYT0    0000 PQ: 0 ANSI: 0
Jan  2 13:06:47 ubuntu kernel: [ 5820.075311] sd 7:0:0:0: Attached scsi generic sg2 type 0
Jan  2 13:06:47 ubuntu kernel: [ 5820.078521] sd 7:0:0:0: [sdb] 312581808 512-byte logical blocks: (160 GB/149 GiB)
Jan  2 13:06:47 ubuntu kernel: [ 5820.079597] sd 7:0:0:0: [sdb] Write Protect is off
Jan  2 13:06:47 ubuntu kernel: [ 5820.079605] sd 7:0:0:0: [sdb] Mode Sense: 27 00 00 00
Jan  2 13:06:47 ubuntu kernel: [ 5820.079611] sd 7:0:0:0: [sdb] Assuming drive cache: write through
Jan  2 13:06:47 ubuntu kernel: [ 5820.082336] sd 7:0:0:0: [sdb] Assuming drive cache: write through
Jan  2 13:06:47 ubuntu kernel: [ 5820.082344]  sdb: sdb1
Jan  2 13:06:47 ubuntu kernel: [ 5820.137699] sd 7:0:0:0: [sdb] Assuming drive cache: write through
Jan  2 13:06:47 ubuntu kernel: [ 5820.137705] sd 7:0:0:0: [sdb] Attached SCSI disk
Jan  2 13:14:58 ubuntu kernel: [ 6311.622584] device eth0 entered promiscuous mode
Jan  2 13:15:02 ubuntu kernel: [ 6315.170077] lo: Disabled Privacy Extensions
Jan  2 13:29:47 ubuntu kernel: [ 7200.557711] Uhhuh. NMI received for unknown reason a1 on CPU 0.
Jan  2 13:29:47 ubuntu kernel: [ 7200.557717] You have some hardware problem, likely on the PCI bus.
Jan  2 13:29:47 ubuntu kernel: [ 7200.557720] Dazed and confused, but trying to continue

The first external Hard Disk (Samsung S2 with 500GB) worked fine.
The second one (Trekstor with 160GB) caused an error which was "WARNING! power/level is deprecated; use power/control instead".
A while after attaching the USB device and copying the file I needed the system froze again showing the NMI error. 
I suppose that it has to do with the energy saving function of attached devices or devices on the PCI bus like my wireless card which I deactivated some time ago.