Hi guys,
David's suggestion for testing is a good one. The GS Wavetable - while convenient because it's on all Windows machines - has some "personality issues"
It does some strange things, including messing up ASIO (which isn't intuitively obvious, but nonetheless true), and other quirky behavior.
Something else that can goof around with MIDI devices (and the GS Wavetable is the Windows internal MIDI device) are programs that use MIDI, such as instant messaging programs (among others) - those little beeps/bells/buzzers are typically MIDI-generated sounds, and so they call on the MIDI driver.
If anything like that is going on on your machine, they can interfere with the proper playback of files. Most newer machines and setups handle things pretty well, but since there are so many different configuration scenarios, I mention this as something to look out for. If someone is having sporadic problems, I typically recommend shutting off anything else that may call on audio or MIDI resources while you're working on files.
ttfn,
Sherry