iwm: loosing sporadically connection - iwm has never been stable

Using a Lenovo E540 laptop equipted with a Intel i7260 WiFi adapter
(iwm0: <Intel Dual Band Wireless AC 7260> mem 0xf0c00000-0xf0c01fff at
device 0.0 on pci3), this adaptor has never been fully operational
since it got supported by FreeBSD on that specific laptop of mine.

I'm running CURRENT(12.0-CURRENT #29 r310417: Thu Dec 22 17:06:29 CET
2016 amd64). The laptop is used in the WLAN environment of the campus
and at my employer's facility and after booting the laptop, the rarest
case is to have a stable connection over the time of work (~4 - 9 hours
a day).  

At this very moment, I receive these messages on the console:

[...]
acpi_ec0: evaluation of query method _Q26 failed: AE_AML_OPERAND_TYPE
wlan0: link state changed to DOWN
wlan0: ieee80211_new_state_locked: pending INIT -> SCAN transition lost
iwm0: iwm_update_edca: called
wlan0: link state changed to UP
wlan0: link state changed to DOWN
wlan0: ieee80211_new_state_locked: pending INIT -> SCAN transition lost
iwm0: add_aux_sta failed
iwm0: iwm_send_cmd: acquiring device failed
iwm0: could not initiate 2 GHz scan
iwm0: iwm_update_edca: called
iwm0: iwm_send_cmd: acquiring device failed
iwm0: iwm_newstate: could not move to auth state: 16
iwm0: device timeout
iwm0: add_aux_sta failed
iwm_init_hw failed 35
iwm0: could not initiate 2 GHz scan
[...]

Ince a couple of days for now, I'm bothered by tons of ACPI messages on
the console as well, but I do not know whether they're linked to the
problem described above:

[...]
WARNING: attempt to domain_add(bluetooth) after domainfinalize()
iwm0: iwm_update_edca: called
wlan0: link state changed to UP
ACPI Error: Needed type [Reference], found [Processor]
0xfffff8000461ab80 (20161117/exresop-111) ACPI Exception:
AE_AML_OPERAND_TYPE, While resolving operands for [OpcodeName
unavailable] (20161117/dswexec-498) ACPI Error: Method parse/execution
failed [\134PNTF] (Node 0xfffff80003f17ac0), AE_AML_OPERAND_TYPE
(20161117/psparse-560) ACPI Error: Method parse/execution failed
[\134_SB.PCI0.LPC.EC.IPCL] (Node 0xfffff80004603f00),
AE_AML_OPERAND_TYPE (20161117/psparse-560) ACPI Error: Method
parse/execution failed [\134_SB.PCI0.LPC.EC.PCLK] (Node
0xfffff80004604000), AE_AML_OPERAND_TYPE (20161117/psparse-560) ACPI
Error: Method parse/execution failed [\134_SB.PCI0.LPC.EC._Q7B] (Node
0xfffff80004604300), AE_AML_OPERAND_TYPE (20161117/psparse-560)
acpi_ec0: evaluation of query method _Q7B failed: AE_AML_OPERAND_TYPE
error: [drm:pid12:i915_hangcheck_hung] *ERROR* Hangcheck timer
elapsed... GPU hung info: [drm] capturing error event; look for more
information in sysctl hw.dri.0.info.i915_error_state error:
[drm:pid1127:i915_write32] *ERROR* Unknown unclaimed register before
writing to 20a8 error: [drm:pid0:i915_write64] *ERROR* Unknown
unclaimed register before writing to 100000 ACPI Error: Needed type
[Reference], found [Processor] 0xfffff8000461ab80
(20161117/exresop-111) ACPI Exception: AE_AML_OPERAND_TYPE, While
resolving operands for [OpcodeName unavailable] (20161117/dswexec-498)
ACPI Error: Method parse/execution failed [\134PNTF] (Node
0xfffff80003f17ac0), AE_AML_OPERAND_TYPE (20161117/psparse-560) ACPI
Error: Method parse/execution failed [\134_SB.PCI0.LPC.EC.IPCL] (Node
0xfffff80004603f00), AE_AML_OPERAND_TYPE (20161117/psparse-560) ACPI
Error: Method parse/execution failed [\134_SB.PCI0.LPC.EC.PCLK] (Node
0xfffff80004604000), AE_AML_OPERAND_TYPE (20161117/psparse-560) ACPI
Error: Method parse/execution failed [\134_SB.PCI0.LPC.EC._Q7B] (Node
0xfffff80004604300), AE_AML_OPERAND_TYPE (20161117/psparse-560)
acpi_ec0: evaluation of query method _Q7B failed: AE_AML_OPERAND_TYPE

[...]


Hope someone can have a look at this.

Thanks in advance,

Oliver Hartmann