I don't know how much you know about this stuff, so if the following post is a bunch of stuff you already know about, it's nothing personal.
I've gotten that exact error with some video files. I did a Google search with "mplayer" and "too many packets in the buffer" - 9,270 results. I don't know anything about making video from video cameras, but it could be something as simple as the file being too big, so you might try using mencoder to re-encode your videos to xvid or something like that which is more compressed and consequently less resource intensive. There's a lot of stuff out there on the web about how to use mencoder, so try a Google search. Avidemux can also be helpful in re-encoding files, but it won't touch anything in *.wmv or *.mov formats (possibly others, I don't know).
If experience is any guide, someone more intelligent and knowledgeable than I will come along soon here and offer better advice. Still, I hope this is helpful in some way.
P.S. Mencoder is a command-line tool that's part of Mplayer. There are probably some graphical front ends for it out there but right now I don't know what they might be. I'm pretty sure Avidemux is in the repos, so you should be able to get it via Gslapt.