I have a similar problem. I'm trying to convert .wma files using WAV writer. I'm running on a 1.4g machine, 500 mb memory, no other apps running. I have ramping, surround sound and auto amp reduction turned off. I've tried both spline and linear interpolation, and glitches occur with both, although they seem more noticeable with spline. I've also tried the WAV writer normalized, with similar results. I've played the .wma file with XM player and don't hear any glitches. Repeated tries do not produce glitches in the same place, but there always are some in some songs. I can see the errors with Goldwave, and it looks like a small section is lost. To my ear they sound like little clicks, not really a missing section of music. I'm using headphones to hear these; I'm not sure they'd be noticeable over speakers.
I don't detect the problem with every file, but some always seem to produce errors. If someone can give me detailed instructions on uploading, I'll be glad to do so. The file is about 6 mb.