Thank You very much for your response. My blacklist-ath_pci.conf file looked like this:
So, there was not ath5k module blacklisted just ath_pci and I've unblacklisted it. Unfortunately that changed nothing - symptoms still present. It looks like some kind of interference for me, because today morning I've noticed that wifi works somewhat better and can achieve 2.5 MB/s under Ubuntu also "stops" are maybe less frequent. Worst situation is in the evening. Ofcourse I've tried changing WiFi channel. Most interesting is that even if this is some kind of interference it's not affecting wifi under win7, where wifi speed and reliability is superb (must say that I've never seen such fast wifi transfers).
# For some Atheros 5K RF MACs, the madwifi driver loads buts fails to
# correctly initialize the hardware, leaving it in a state from
# which ath5k cannot recover. To prevent this condition, stop
# madwifi from loading by default. Use Jockey to select one driver
# or the other. (Ubuntu: #315056, #323830)
Can You please tell me what is in your file blacklist-ath_pci.conf ?