This is obviously a network issue since neither Safari/Chrome or Flash/HTML5 are not working for you. It is interesting that you are have having the problem on both wired and wireless connections, that would lead one to believe that it is a modem/router or throughput issue. I would try one of the following.
1. Reboot the comhem modem and see if that improves things.
2. If that doesn't work, do a factory reset on the modem
3. Consider changing your DNS to googles 8.8.8.8, and 8.8.4.4 dns servers.
You say that you have a good connection. What speed ratings are you getting from bredbandskollen.se and speedtest.net. Best to run these tests when you find Youtube is not working for you. It is possible that something else is using the network when you don't expect it. A good culprit is your iphone icloud backup and photostream sync, they can soak up all your upload bandwidth, killing your download speed. Put them all in airplane mode and test again.
Considering investing in Mac App Store - iNet, it's a great tool to let you see what is happening inside your network.
