I have had basically the exact same problem. It appears to have stopped happening now though. I'll post what might have helped fixing it, then my own specific issues and observations.
TL;DR - Try updating DirectX / other 'background' software?
What I did to fix:
1) Allowed osu! through firewall (this was not done/prompted with the new installer)
2) Ran osu! as admin
3) Updated Wireless adapter and Graphics card drivers
4) Had Windows 8.1 fully up to date
5) restarted computer
6) Installed steam and some games
During Steps 1-5 I still had this issue. After Step 6 the problem stopped (so far
). I assume this has something to do some software/settings that is done with Steam/other Games installation but was not with osu! I don't really know how osu! requires to run, all I know is that I don't have this problem know.
Below are my observations. I was going to create a new post asking how to fix this but thought I should post here to help others since this one came up on google when I searched it.
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Situaution:
--> Playing Osu fine using Windows 8.1
--> Complete clean install of Windows 8.1 (computer was running slow)
--> Wireless router would disconnect and now be avaliable for around 10secs each time I played a BEATMAP in osu!
--> Would reappear again and I could reconnect. This would not occur automatically even if I ticked the box to 'connect automatically' previously.
Observations:
- Disconnection only occurred when I played a beatmap. I could start osu! fine and have it idle without this happening.
- The router itself didn't reset. I still had connectivity on my phone to it during the downtime.
- I had no disconnection issues with other games (could play a game of LoL fine).
Conclusion:
- Might be a problem with submitting scores with Bachobot (?).
- I changed no setting on my router so its probably wasn't an issue with ports or the likes.
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That's about it. Sorry for replying to a old post. I don't know what fixed it specifically but I hope this information will help someone in a similar situation. This was bugging me for a while as I usually play osu! while waiting for mates in LoL. The router would dc and I would dc from the LoL client as well.
EDIT - Experencing the problem again. Was working fine yesterday