Help with Bandsaw Blade... Drift?

New woodworker here, trying to make a bandsaw box like [David Picciuto](https://youtu.be/lLaEZO8bJPY). I tried to make a nice straight slice off the back of the workpiece, with the grain. 14" Grizzly from 1986. I put on the 0.5" 10tpi blade. I set up a fence accounting for blade drift (freehanded halfway down a straight line in scrap wood, set the fence against the edge), and tested it with a small piece of scrap - perfectly straight. When I ran the big box block into the setup, it did fine until about halfway through, but then went all squirrely on me. The blade started driving the workpiece off the fence and diving closer to the edge, and wound up about 1/8" from where I wanted it out the other side. Something was making sounds that weren't right, and on the second half of the cut, I wound up with a bunch of wavy channels, rather than a nice smooth cut.

As I say, 0.5" 10tpi blade, which I *believe* I'm able to tension enough. The crank/spring is maxed out, but there isn't much deflection in the blade. The saw is set up with good guide bearings and cool blocks, all of which were adjusted correctly to the blade. I tried to save the workpiece by resetting the fence to square off the back, but the same thing happened - the blade wanted to dive inward toward the fence during the second half of the cut.

I'm very frustrated, because I did everything I could to prevent this, and don't know what I did wrong. AND I ruined a nice piece of walnut. The workpiece is a weird rainbow-shaped glue-up, could that have something to do with it?

[Here are three pictures of the slices that came off the workpiece](http://imgur.com/a/kaVhH)

Help! Thanks, joe in mpls