I've noticed with my provider that some channels are more stable than the others. For example, it seems like popular channels like movies, sports, etc are less stable and tend to suffer from buffering more than let's say Science channel. Is it just me, or is there a reason for that? Load?
Load is probably the biggest reason.
One thing to remember is that most of these IPTV providers are not licensed to sell access to these channels, and use a combination of authorized and unauthorized streams to get you the video content they promise.
They can not easily create a second stream, however, and if too many people are trying to watch, let's say, the WWE event for the week, then it can cause the server to overload.
But that stream of the Science channel is perfect, because less people care about documentaries on tree frogs.
Since the streams themselves are on different servers, the tree frog documentary is not affected by the heavy load on the WWE stream.