Slow hard drive or slow LAN ?
I would test both.
There's programs and instructions for doing both in my server thread if you need them.
I would check your LAN speed and also run a HDD test.
You could just try copy and pasting a problem movie and check your speed. If its over 50mb/sec then you know neither are your problem. In which case XBMC crappy player is the culprit. Double check settings or re install XBMC.
Have you tried other external players in the same problem movies ?
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."