It's difficult to say. It depends on the source of the problem. When the slow playback occurs you should open the Task Manager to see the CPU/memory usage. It's possible the app you're using to view the file has a memory leak. I've experienced this with various softwares (bittorrent clients, winmx, mp3/tag studio). The longer I kept them running, they eventually reached a point of instability.
I'd recommend you try a different viewer: bsplayer, zoomplayer, media player classic.
What codecs do you have installed? Did you use something such as mega codec installer that installed alot of codecs you've never heard of? If so, that could be the root of your problem; never install a codec unless you know what it does.
I have the following installed
AC3Filter 1.01a RC5
DivX Pro 5.2.1
ffdshow-20041012 (divx 3, 4, 5 options diabled during installation)
ffdshow handles xvid, mpeg, m2v, playback.
divx pro handles divx 3,4,5 playback.
Unless you plan on doing divx encoding I'd recommend you just install ffdshow with the default settings. That provides the cleanest setup.