To me, it clearly looks like something is moving around or flexing. Properly installed feather boards will not produce the result you're getting. I don't think grain has anything to do with it and I don't think it's the relief of built up stresses in your wood. Assuming your fence and featherboards are properly installed and rigid ... the first thing I would look at is that blade. You said it's only .050" thick ... seems pretty skinny to me ... my Freud thin kerf blade body mikes out at .068". A skinny blade is certainly gonna flex more and heat up quicker. I'd also look at the blade to arbor alignment, although I doubt it could be so bad as to cause your problem ... BTW, I use a Veritas saw blade truing disk and swear by the thing. I'd try a couple-o-strips using a standard 10" rip blade and see how it goes. I'd also suspect ... a loose ... or loosening blade ... bent arbor, lousy bearings etc ... anything that can effect alignment. As others have suggested I'd use a kerf splitter to keep the strip from vibrating around and rubbing on the back side of the cut. If you're not using a kerf splitter and you have a lot-o-vibration, I think you could get a grossly wavey cut like that if you used a slow feed rate.

Ken Blanton
... The Outer Island RULES! ...