Think of the buffer in the Wii like a swimming pool.
The stream of video is the stream of water filling up the pool, the process of playing the video is an outflow of information emptying the pool. If the pool isn’t being filled quickly enough it will empty and the outflow will stop. When you pause the video you are switching off the outflow to let the pool fill up again.
Films with higher bitrates and resolutions need a big pool, because a lot of water (info) needs to flow out more quickly. Higher resolutions have other implications too, ie the graphics card cant actually process all that info quickly enough.
The Wii has a pretty small pool (in comparison with a PS3, xbox 360, most decent PCs, etc) so we need to fill it faster. That is why ther LAN adapter is always preferable for both SMB and net streaming, it is way faster than Wifi and with less overheads and free from other issues such as interference and signal strength. This counts for the wii and any other computer or attached storage.
You can also check the filesize of the film: an hour and a half long move at 1.5GB will have a much higher bitrate than the same movie at 700MB in size, and so the Wii might struggle with it a bit.
