While this can be different for everyone, the most common reasons are:
1. Interfering Software
You have software running (often without knowing) that is using enough of your upstream bandwidth to interfere or is maxing out your routers total connections. This could be a more obvious peer-to-peer program like Bittorrent or FrostWire/LimeWire, or a less obvious one like Steam or Skype. To fix this you will need to completely shut off any such software. Use the MSCONFIG utility to disable them from starting up, then restart your computer. If you have other computers on your network, turn them all off just in case the problem is on one of those machines instead. Later you can power them back on one-by-one to find out which one is causing the problem. Also try restarting your router by unplugging the power cable and plugging it back in; a power cycle like this clears out any cached connections and resolves a number of issues that might be plaguing your router.
2. Network Congestion or Line Problems
In some cases, the connection from your computer to your router can be the cause of this issue. To confirm this as the problem, plug your computer directly in to the modem, without the use of the router. If you have a combined modem/router, ensure you're connected by a quality CAT5e or CAT6 cable and disconnect other computers, just in case they are interfering. You can plug them back in later once you've confirmed the problem.
Often the problem can be caused by periodic interruptions due to poor signal quality in the lines from your computer to your Internet Service Provider (ISP). This is the cause of the problem much of the time someone is experiencing lag or disconnects. Check to be sure you can upload at full bandwidth consistently using other software like FTP or any file sharing website. If your ISP has a speedtest, keep running it repeatedly and see if the current speed indicator is fluctuating while uploading. To test the connection using a 3rd party source, run a speedtest on speedtest.net and check to be sure you're getting your full bandwidth as promised by your Internet Service Provider. If you're not, contact them with the results and request to have the problem repaired.
3. Limited Upload/Upstream Bandwidth
Although many high speed internet connections offer fast download speeds, the upload speeds can be heavily limited. If your internet connection's upload speed maxes out at 128kbps and you're streaming at 128kbps, there may not be enough wiggle room to stream smoothly. In this case you will need to lower your streaming bitrate until it falls well within your total upstream bandwidth capabilities, or you will need to upgrade your Internet plan with your ISP to a faster upstream connection.
4. Location / Distance
You're too far away from our server. In some cases, if your computer is on the other side of the globe, the latency and number of hops between your computer and our server keeps the connection from working consistently. Unfortunately this one cannot be fixed - you will need to find a more local streaming provider. If you're geographically fairly close to our servers (anywhere in North America or Western Europe) then this should not apply to you.