On another machine that I have access to also running Vista64, I have no issues at all with songs stopping. I do still have the gapless broken issue regardless of sample rate or whether or not I force the sample rate to be the same for all formats. This PC uses uses onboard sound. (SoundMax HD Audio).
What does the "Output" text in the "General" info window say when using WASAPI output? If there are 2 lines, try setting the sample rate to the number in the 1st line and enable the "Apply sample rate to all file formats" option. That tells XMPlay to always output at that rate (resampling if necessary), so it doesn't have to bother asking the output device to change its rate if the file uses a different rate.
Note that gapless output (as well as crossfading) requires that the output rate doesn't change.