The proposed actions do not help with this issue. The WIFI still drops, and the iMac is no longer able to see any wireless networks.
But: I have a feeling that the issue is related to Time Machine. I restarted the iMac, and the wireless network was connected. I opened a web page with success. Then I manually started a Time Machine backup, and the second I did that the wireless connection was gone. This is also seen in the kernel log as follows:
WAKE UP
May 18 18:45:07 imac kernel[0]: Wake reason: EHC2
May 18 18:45:07 imac kernel[0]: Previous Sleep Cause: 5
May 18 18:45:07 imac kernel[0]: The USB device HubDevice (Port 1 of Hub at 0xfa000000) may have caused a wake by issuing a remote wakeup (2)
May 18 18:45:07 imac kernel[0]: The USB device Keyboard Hub (Port 3 of Hub at 0xfa100000) may have caused a wake by issuing a remote wakeup (3)
May 18 18:45:14 imac kernel[0]: The USB device Apple Keyboard (Port 2 of Hub at 0xfa130000) may have caused a wake by issuing a remote wakeup (3)
May 18 18:45:14 imac kernel[0]: 589.681427: performCountryCodeOperation: Not connected, scan in progress[0]
May 18 18:45:14 imac kernel[0]: 589.681440: performCountryCodeOperation: No scan in progress, disconnect VAP
May 18 18:45:14 imac kernel[0]: 589.681442: setDISASSOC from ATH_INTERFACE_CLASS disconnectVap
May 18 18:45:14 imac kernel[0]: HID tickle 389 ms
May 18 18:45:14 imac kernel[0]: Ethernet [AppleBCM5701Ethernet]: Link up on en0, 1-Gigabit, Full-duplex, Symmetric flow-control, Debug [796d,2301,0de1,0300,c5e1,3800]
May 18 18:45:15 imac kernel[0]: en1: 802.11d country code set to 'DK '.
May 18 18:45:15 imac kernel[0]: en1: Supported channels 1 2 3 4 5 6 7 8 9 10 11 12 13 36 40 44 48 52 56 60 64 100 104 108 112 116 120 124 128 132 136 140
May 18 18:45:15 imac kernel[0]: [ffffff8019da5400][BNBMouseDevice::init][70.12] init is complete
May 18 18:45:15 imac kernel[0]: [ffffff8019da5400][BNBMouseDevice::handleStart][70.12] returning 1
May 18 18:45:15 imac kernel[0]: [ffffff801feff600][AppleMultitouchHIDEventDriver::start] entered
May 18 18:45:15 imac kernel[0]: [ffffff801c0e6300][AppleMultitouchDevice::start] entered
May 18 18:45:15 imac kernel[0]: [ffffff8020178000][BNBTrackpadDevice::init][70.12] init is complete
May 18 18:45:15 imac kernel[0]: [ffffff8020178000][BNBTrackpadDevice::handleStart][70.12] returning 1
May 18 18:45:15 imac kernel[0]: [ffffff8019d7ca00][AppleMultitouchHIDEventDriver::start] entered
May 18 18:45:15 new-host kernel[0]: [ffffff8020a78d00][AppleMultitouchDevice::start] entered
May 18 18:45:15 new-host kernel[0]: en1: BSSID changed to 10:9a:dd:85:1e:89
May 18 18:45:15 new-host kernel[0]: AirPort: Link Up on en1
May 18 18:45:15 new-host kernel[0]: en1: BSSID changed to 10:9a:dd:85:1e:89
May 18 18:45:30 imac kernel[0]: en1: BSSID changed to 10:9a:dd:85:1e:8a
AT THIS POINT I MANUALLY START THE TIME MACHINE BACKUP
May 18 18:46:32 imac kernel[0]: AppleSRP started.
May 18 18:46:33 imac kernel[0]: 675.548516: ath_reset: unable to reset hardware; hal status 17
May 18 18:46:35 imac kernel[0]: 676.846986: ath_chan_set: unable to reset channel 136 (5680Mhz) flags 0x10100 hal status 17
May 18 18:46:36 imac kernel[0]: 678.269657: ath_chan_set: unable to reset channel 140 (5700Mhz) flags 0x10100 hal status 17
May 18 18:46:38 imac kernel[0]: 679.696815: ath_chan_set: unable to reset channel 100 (5500Mhz) flags 0x20100 hal status 17
May 18 18:46:40 imac kernel[0]: 681.724471: ath_reset: unable to reset hardware; hal status 17
May 18 18:46:47 imac kernel[0]: AirPort: Link Down on en1. Reason 16 (Group Key Handshake timeout).
BAM! WIRELESS NETWORK DOWN!
Can anyone try this too? Wake up your iMac, check that your connection is up, and then start a Time Machine backup and look for the connection to go down immediately?